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Deep hum, not drone, cruising

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.
Interesting thought.
 
'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?
Yep.
Research Helmholtz resonator for exhaust. If you get the length right, drone will be gone, tone and volume stay the same.
A little math, maybe, a little experimentation.
Or, you could just put on some bigger, longer, quieter mufflers.
 
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