Weird harmonic drone/vibration at ~2200RPM.

I have this very annoying drone/vibration at 2200RPM. It sounds through the exhaust, but hard to nail down. It's in drive/driving and (a very slight lesser degree) in park too. It reminds me of a driveshaft that's out of balance/pinion angle off. That type of on/off drone. The car runs/drives excellent. In fact best it ever has, except this drone. Goes away as soon as it goes above/below that rpm. I'm pulling my already bald head hair out!

...It's a stroked 400/727/8.75 and a 67 Coronet.

Same place, same RPM every gear?
Or same place, same MPH every gear?

Same RPM

Check to see if your balancer is tight on
the crank(not a loose fit)..pull your belts
off and try it

Make sure the engine/trans. and exhaust are all clear of everything. If you have some super tight clearance somewhere it usually will rear it's ugly head like you describe. Otherwise could be a plethora of stuff. I would drop the belts like Mike said and try that.

Ok, I'll try some of the suggestions. It's not a vibration I feel. It's one I hear, like an exhaust drone, but vrrrrrrr vrrrrrrr vrrrrrrr vrrrrrrr vrrrrrr. Not a constant drone, pulsating. Only at 2200. Weird.

I have heard that and know what you mean. I had it in a '76 Dodge Ramcharger. Had dual exhaust but could have been anything as I never chased it.

That sounds like a bearing to me.

Will it do it parked and held at 2200?

If so it's engine or input side of the transmission related.

What does it do if you run it past that speed on the road, put it in neutral and let it coast back down?

If it does it then it's probably a tailshaft bushing or
a wheel bearing if it's the kind of vibration that I think you're describing.

Kevin

I suspect some resonance in the exhaust system.
For a test, try hanging some weight on the clamps (small bar bell using bailing wire) - if it changes that's it.
The cure may be as simple as moving a clamp, adding a clamp, or weakening a clamp (making it more flexible).
Last: move your X-over or push the mufflers back a bit.

He said it did it in park also

I believe its in the exhaust also... add any weight to
it and see if it changes the rpm point

I agree - check the exhaust. I had a similar issue caused by a poorly fitting slip joint on one of the header tubes.

The cyclic nature of the vibration points to a 'beat frequency' problem. When two vibrations that differ in frequency by small amount combine the result is a vibration that comes and goes as the vibrations alternatively cancel each other or add to each other. With exhaust systems you can get into changing parts or playing with X or H pipes but I'd look into the tune-up and make sure it isn't lean or rich at that point before doing anything else. I've seen several cases where lean cruise conditions cause that annoying 'drone' sound.

Do you have Drone (Flow) Master mufflers?

If all else fails, you may have a short straight section of both pipes (but may not be in the same place) where you can insert an "empty bottle": a 3" ID X 6" can to change the resonance by adding another frequency.

No flow masters. I'll try weights hung on clamps and lean /rich conditions today. I'll also check the headers and exhaust.

I checked those ideas. Still there, no change with weight. Its coming from front of car, under hood. Can a fan clutch, fan, pulley, etc..possible? Balancer seems ok.

I chased a drone vibration for a long time, was under the hood one day, was at the front of the car, leant in to blip the throttle, so rested my hand on the alternator as I blipped it, bingo!!!! Right under my finger tips alternator was buzzing, found out the armature was out of balance, quick call to RockAuto, replaced it and smooth as silk!!