I am getting MAZDATRIX driveshaft tomorrow, but I wanted to be sure that there are no other suspects. Frame vibration sounds as if frame was being hit with rubber hummer. It starts @ 50MPR and slightly increases with speed. It starts some time at higher speeds and not right away. Once it starts beating, it gets worse when foot is off gas pedal and remains about the same or a bit worse when in neutral. Could it be something else that the driveshaft? I have about 70-80K on Mazda OEM shaft. The car has 230K on it.

 

it could be a wheel bearing, or drive shaft.

get under the car, and try twisting the shaft, there shouldn't be any play. also, try and corner hard, see if the sound gets quieter, or maybe even goes away..this would suggest a wheel bearing.

 

Have you check'd your rear end? That same thing happen'd to me with my 3.9, all my fluid leak'd out and that baby howl'd and shook a shitload once I put it in neutral and was coasting.

 

Thanks! I have checked wheel bearings (wheels are balanced) but did not check diff. fluid. However, it beats both in gear and in neutral with only a slight increase in beating magnitude when decelerating or when in neutral. I will check the fluid though, thanks again. Would CV joints cause beating noise like that?