Strange resonating noise at certain RPM's??

[Robe81]

Getting a very distinct loud, deep, noise at certain RPM's. It seems to get louder and more pronounced and then kind of levels off as I increase speed in the same gear. It definitely goes away when I shift into the next gear (until it shows up again at another speed) so it cant be a wheel/tire issue. Below is the range I usually feel it:

2nd gear = about 20-25 mph
3rd gear = about 35-40 mph
4th gear = 60ish mph

I have no tachometer so cant tell what the rpm range is that its occurring in. The sound can only best be described as a sort of...

wa...wa...waa...waaaa...waaa...waaaa...WAAAA...WAAA...WAAA..WAAAAAAAAAAAAAAAAAA. Then I shift and its over...until the next gear range when I hear it.

Again its a very deep noise. Nothing high pitched at all.

Two shops have looked at it and they swear its an engine tuning problem, but neither is willing to attempt a fix.

Vehicle is a 1969 Roadrunner 383 4 spd. Any advice or help would be appreciated.

Thanks!

 

[Hemi Rebel]

What kind of exhaust do you have? Headers or manifolds? What size?

- - - Updated - - -

Maybe also could be trans input shaft bearing.

 

[Robe81]

No headers or aftermarket manifold. Just flowmaster mufflers.

 

[HEMI-ITIS]

It's the deadly flushmaster drone:blob1: