Need help isolating a weird vibration

[Jonnyuma]

I'll try to keep this as concise as possible, but unlike the stupid Aamco ads on tv ("my car goes grunk grunk"....."its ya modulataaah". Thanks genius), it takes a lot of info to diagnose something as nebulous as a "weird vibration". I have a rythmic vibration (if this was a Harley and not a 76 Cordoba chicks would be buying tickets to ride this machine) that comes on at 1900 rpm. It goes: rrruuuuuummmmm rrruuuuummmm. rruuuuummmm, like driving over fine lines grooved into the pavement perpendicular to the direction of travel. I'm feeling it through the seat, not the steering wheel...full-body shiver. I give it a little gas and it speeds up; rruum rruum rruum. Lift and it slows waaay down. Totally disproportionate to both engine and road speed. Ive tried to duplicate it in the two lower gears and get no vibe at the same rpm. In nuetral I THINK I'm getting it, but its very faint, almost unnoticable which makes me think I could be imagining it. 0 vibration in P at the same rpm. My best guess at this point is that its behind the engine, maybe the trans mount? Pinion angle? Maybe its mah mahdjulatah (well...thanks, Django. But I'm almost 100% sure its not). I really dont know and my current situation as a broke-***, apartment-dwelling, bachelor-to-be doesn't allow me much chance to diagnose this gremlin. On the plus side, I'm driving this thing 40 miles EVERY day so if any of you guys have any tips on helping me diagnose this thing there will be no "I'll have to wait til the weekend before I can drive it"...this dude is taking me to work every dang day. So far so good, but the vibration is driving me a little nutty and makes me worry 'bout damaging a major component.
Thanks for help. Love FBBO.

 

[krowbar]

Check rear diff lube? I have been having trouble on a 95 gmc truck. Replaced carrier bearing ( thinking maybe drive shaft ). Replaced U joints, new shocks, lubed leaf springs, lubed front bushings,alignment..... It's a PITA but mine is more of a slight jumpy feeling in the rear. I don't hear any noise. I know it's not the same description but maybe it will give you some ideas in diagnosing.

 

[kb73rr]

You are getting a harmonic vibration. That happens when two or more seperate parts are vibrating at similar frequencies (or multiples thereof) and their vibrations "overlap" and reinforce each other. That's why there seems to be no direct correlation to rpm or vehicle speed.

The main source of the vibration is indeed likely to be engine, exhaust, or driveline related. Any of those are going to be hard or impossible to completely eliminate. However, there will be a second part that is resonating with that and making the noise. If you can find the noisy part and dampen it or change it's frequency you can stop the harmonic. I've been chasing something like you are describing for a while now. It was driving me crazy because it would come and go. In fact I was almost ready to record the sound and post it on here. Then I figured it out. In my case the part making the noise was the gas tank vibrating against the trunk floor. It would vibrate at different frequencies depending on how full it was. I installed another cushion on top of the tank and it fixed the problem.

You might also check your deck lid (skin vibrating against inner support structure), package tray, headliner bows, interior trim, the heater box, or the door cards. It might help to have someone ride along with you (both front and back seat) to see if they can help you find the offending item.

Besides being annoying harmonic vibrations could eventually cause fasteners to come loose or fatigue cracking so it is a good idea to fix it.

 

[67 coronet]

if you can feel it in the seat of you pants and not in the steering it is more then likely in the drive train.(trans on back)check for a broken trans mount and any loose parts first.including u joint movement.make sure your rear diff is up on fluid as well.easy stuff first.

 

[Jonnyuma]

I haven't checked the diff's oil level yet. I had ruled it out because I changed/topped it up a yr or so ago but it can't hurt to check it again. Last time I had it up in the air I looked at u joints, drive shaft, and torque converter bolts and couldn't find anything gone horribly wrong. I will be changing the trans mount and, in fact, asked for recommendations and suggestions down in the 75-79 section.
This vibration isn't something I can actually hear, apart from some interior squeaks, but those are secondary...results of the original vibration.

 

[Meep-Meep]

You are getting a harmonic vibration. That happens when two or more seperate parts are vibrating at similar frequencies (or multiples thereof) and their vibrations "overlap" and reinforce each other. That's why there seems to be no direct correlation to rpm or vehicle speed.

The main source of the vibration is indeed likely to be engine, exhaust, or driveline related. Any of those are going to be hard or impossible to completely eliminate. However, there will be a second part that is resonating with that and making the noise. If you can find the noisy part and dampen it or change it's frequency you can stop the harmonic. I've been chasing something like you are describing for a while now. It was driving me crazy because it would come and go. In fact I was almost ready to record the sound and post it on here. Then I figured it out. In my case the part making the noise was the gas tank vibrating against the trunk floor. It would vibrate at different frequencies depending on how full it was. I installed another cushion on top of the tank and it fixed the problem.

You might also check your deck lid (skin vibrating against inner support structure), package tray, headliner bows, interior trim, the heater box, or the door cards. It might help to have someone ride along with you (both front and back seat) to see if they can help you find the offending item.

Besides being annoying harmonic vibrations could eventually cause fasteners to come loose or fatigue cracking so it is a good idea to fix it.

An excellent post clearly displaying the understanding of vibration! It does sound like the car found the natural frequency of a component or two.

 

[idrivemopar]

I would check drive line, and I say this as your timing couldn't be better. I suspected I had a drive line issue for years, strange random vibrations and what not, turns out, my drive shaft was installed backwards, out of balance, and the yolks were bad, they had rounded out holes for the u-joint cups and you would never know it unless you checked it with a micrometer. So, even if it doesn't look bad it can still be bad.

 

[Jonnyuma]

On my way home from work tonight I found what I thought to be a perfectly viable way to avoid (not fix, yet) my harmonic vibration...I just skip the, uh...what do you call the opposite of a "sweet spot"? Sour spot? Wet spot? Eeww, no thanx. Whatever it is, I just kinda blow past 1900 rpm and cruise at around 2100. If I don't let the vibration get started, it just isn't there. It does have the unfortunate side effect of putting me at odds with the local constabulary, something I try to avoid... almost always. See, about 90% of my daily commute is a 45 mph 2-laner. 1900 rpm is right where I spend that whole drive (46-47 mph). 2100 puts me up around 52-53, citation territory. I think we can all agree that 7-10 mph over the suggested speed limit, especially for as noble a cause as HVA (harmonic vibration avoidance) should be excusable in the eyes of the law. I'm thinkin' the Revenue Appropriation Technicians (governments just LUV acronyms) might see it otherwise.
I couldn't get it (the vibration) started in L1 or L2, could there be something internal? The D band, maybe? Auto trans' are black magic to me, although I'd like to get to understand em better. I'm still leaning towards driveshaft/u joint/pinion angle buggery. It'd be great if I could mount a little video camera down there, maybe catch the gremlin in action.
I apologize if my posts are too long, and thanks to youz that have stuck with it and offered help. I like to talk and sometimes I like to type...and sometimes I just have a problem GETTING TO THE FREAKIN' POINT. I appreciate the suggestions and like the raggedy-*** guy on the freeway onramp's cardboard sign says; "Anything Helps"....although the scamp turned down the sandwich and apple I offered 'im. go figure.

 

[Meep-Meep]

You can get a protractor at a hardware store and check drive line angles, and correcting those angles might be as simple as replacing a tranny mount. Also inspecting anything that rotates under the car is not a bad place to start. I won't rule out worn tranny components either (including converter) but start with the easy stuff first.

 

[Jonnyuma]

Gentlemen, I believe the correct answer was...drumroll...tranny mount.
I changed it out today and things seem much, much better. I didn't have time for a complete shakedown, but the trip from here (home) to my favorite beer cooler (c-store about 2 mi away) was vibration-free. The trip in to work tomorrow will tell the tale, but for now I'm calling it...victory.

Thanks for the help.

 

[soundhd]

I would have the driveshaft checked for balance......very easy to do.......Had a weird vibration problem with my 70 RR when I first bought it.......was told to have the driveshaft checked.....was common problem with Mopars.......yep, it was out of balance......balanced it and all is fine.......at least if you have it checked it you can take it off the list.........

 

[68 Sport Satellite]

Excellent post!

You are getting a harmonic vibration. That happens when two or more seperate parts are vibrating at similar frequencies (or multiples thereof) and their vibrations "overlap" and reinforce each other. That's why there seems to be no direct correlation to rpm or vehicle speed.

The main source of the vibration is indeed likely to be engine, exhaust, or driveline related. Any of those are going to be hard or impossible to completely eliminate. However, there will be a second part that is resonating with that and making the noise. If you can find the noisy part and dampen it or change it's frequency you can stop the harmonic. I've been chasing something like you are describing for a while now. It was driving me crazy because it would come and go. In fact I was almost ready to record the sound and post it on here. Then I figured it out. In my case the part making the noise was the gas tank vibrating against the trunk floor. It would vibrate at different frequencies depending on how full it was. I installed another cushion on top of the tank and it fixed the problem.

You might also check your deck lid (skin vibrating against inner support structure), package tray, headliner bows, interior trim, the heater box, or the door cards. It might help to have someone ride along with you (both front and back seat) to see if they can help you find the offending item.

Besides being annoying harmonic vibrations could eventually cause fasteners to come loose or fatigue cracking so it is a good idea to fix it.