Deep hum, not drone, cruising

[Dino1147]

'69 Charger with '71 440 30 over. HP manifolds, TTI 2.5" exhaust with H pipe, and Dynomax/Walker 17748 Super Turbo mufflers. Stock style exhaust tips that slip over the pipe. Edelbrock Pro Flo 4 EFI and Crower 32242 Crower 32242 Crower Compu-Pro Performance Level 3 Camshafts | Summit Racing
Johnson lifters, Comp cams double roller timing chain and 911 valve springs. Otherwise stock. Stock '73 727. 8 3/4 rear with open 3.23:1. 27.8" tall tires.

The car runs great but I have this pretty loud and annoying hum from ~1750-2100 rpm, which is most of my driving rpm range through town. Stop and go traffic it sounds great, same when getting the rpms up a little. Light throttle doing 55 it sounds great and not at all too loud, same doing 70 or more, but it will still growl when you push the go pedal down. I tried Dynomax Ultra Flows 17513 and it has the same problem.

I've never had a real drone, I don't think. From what I can tell from others, it's a drill in the skull type of deal. The closest I've encountered like that is when I'm on the highway, with any car, and I close the front windows but not the rear, and it feels like your eardrums are gonna blow.
This is a constant deep hum and goes away as soon as you let off the throttle. It doesn't give you a headache or make anything tremble, it's just obnoxious.

I have some 12" Cherry Bombs I can mount in the tail pipes but I'm not sure that would fix it, and I'd hate to cut the pipes only to find out it was for nothing. Alternatively I could try Walker Quiet mufflers but I only want this hum gone, not the sound it makes throughout the rest of the range.

Any ideas what's going on and how to remedy it?

 

[Nirvana]

That’s drone, add resonators to the tail pipes, not cherry bombs.

 

[Charged440]

I ended up using the longer case 17748, resonators work too, but I had tried them an another car and it wasn’t enough for me personally.

 

[Dino1147]

That’s drone, add resonators to the tail pipes, not cherry bombs.

Aren't they basically the same? The bullet style?

 

[Dino1147]

I ended up using the longer case 17748, resonators work too, but I had tried them an another car and it wasn’t enough for me personally.

That's what I have. I wrote the wrong number. I fixed it.

 

[steve340]

It could be the exhaust mounts transmitting vibration.
How is the exhaust system mounted?

 

[Dino1147]

It could be the exhaust mounts transmitting vibration.
How is the exhaust system mounted?

With the TTI provided hangers, bolted to the stock floor brackets. Apart from this it is bolted to the exhaust manifolds on end end, and the tip is suspended from the stock rubber block on the other end. When I first installed it, one of the pipes would touch the frame rail, and that made a lot of racket.

 

[steve340]

That should be OK. An experiment to see if you can change the vibration/noise is let one of those rubber straps off and go for a short ride.
What about engine and transmission mounts?
Something else that can rub on the body?

 

[Dino1147]

That should be OK. An experiment to see if you can change the vibration/noise is let one of those rubber straps off and go for a short ride.
What about engine and transmission mounts?
Something else that can rub on the body?

Nothing I can think of but I'll double check.

 

[Nirvana]

Aren't they basically the same? The bullet style?

Round metal and for your exhaust… in all other ways they are not the same.

A cherry bomb is a cheap performance muffler, its goals are to be loud and flow more(most don’t) compared to your standard muffler.

A resonator targets specific, often high-pitched, frequencies and smoothes them out.

 

[Dave6T4]

How about your fan? A large direct drive fan can cause a hum at highway speeds.

 

[Dino1147]

Round metal and for your exhaust… in all other ways they are not the same.

A cherry bomb is a cheap performance muffler, its goals are to be loud and flow more(most don’t) compared to your standard muffler.

A resonator targets specific, often high-pitched, frequencies and smoothes them out.

Gotcha. Thanks!

Ideally the exhaust tips go straight onto the resonators, but I'll need 2.5" ID to fit over the pipe and that's the same size as the tips ID.

 

[Dino1147]

How about your fan? A large direct drive fan can cause a hum at highway speeds.

It hasn't changed from before when I drove with headers and a different exhaust. It only makes noise under 2200 rpm.

 

[RR Fan Dan]

It hasn't changed from before when I drove with headers and a different exhaust.

So your car didn’t used to drone, and then you changed exhaust and it started to drone? If that’s the case, then that’s a valuable piece of information there.

 

[Dino1147]

So your car didn’t used to drone, and then you changed exhaust and it started to drone? If that’s the case, then that’s a valuable piece of information there.

Yeah I could have mentioned that little tidbit...

Before this, as I bought it, the car had rusty unknown brand headers, 2.25" true duals, and some type of Walker mufflers. No H or X pipe. It had a purple cam that was happiest at 3000 rpm and above, and a carb. That was probably the only difference.

 

[RR Fan Dan]

Yeah I could have mentioned that little tidbit...

Before this, as I bought it, the car had rusty unknown brand headers, 2.25" true duals, and some type of Walker mufflers. No H or X pipe. It had a purple cam that was happiest at 3000 rpm and above, and a carb. That was probably the only difference.

So what exactly did you change to cause the drone? Full exhaust including headers and added a H or X?

 

[Dino1147]

Specs are in my initial post.

 

[Some Car Guy]

Round metal and for your exhaust… in all other ways they are not the same.

A cherry bomb is a cheap performance muffler, its goals are to be loud and flow more(most don’t) compared to your standard muffler.

A resonator targets specific, often high-pitched, frequencies and smoothes them out.

This exactly. A true resonator will not effect flow, it will target the sound.

My 69 GTX has vibrant round can style resonators. The drone described by the op has been in many of my cars. I decided I would spend whatever it took to have this car be quiet. The big super turbo dynomaxs, x pipe, vibrant resonators, stock style tips in 2.5 to match the size engine to tips, and new stock hardware made for 2.5 to hang it all and I have zero drone and buzz now. Gone.

 

[Resto_1]

Ran into a brad droning problem with a Super Bee I recently bought. 4-speed car with factory exhaust pipes and glass packs. Droning took the pleasure out of driving the car at steady speeds. Bought a pair of repro 2781300 Hemi mufflers. Still has an anoying drone, but not nearly as bad. Great tone outside the car when accelerating. I'm also on the resonator line of thinking, but hoping I'm not throwing good money after bad. Many moons ago I had a similar car that I put a pair of factory Hemi mufflers on, no drone.

 

[Quikshft]

Happened to me years ago - a Dart had an annoying drone at cruise, it could even be replicated sitting in the driveway. Turned out to be a bad transmission mount. I put up with if for a long time before figuring it out.