Anyone else have issues with a buzzing noise in 4th gear only?

Ive got an 87 TII trans which has about 90K miles on it,and has worked great for nearly 6 years.But in the last couple years its developed a strange buzz in 4th gear when cruising.It goes away if I coast or accelerate, and its only in 4th.
No shifting problems or vibration,its not the shifter,mount,U-joints or rearend...all that stuff has been replaced/trouble shot in the past.Its just a loose sounding,mettalic buzz around 3200-3500 RPMS in 4th under cruise.Always ran Redline MT90.

Im not familiar with any common failure points of the ribcase trannies,so Im looking for any experiences anyone has had with their weak points.In 4th gear,the entire drivetrain is essencially locked together as a solid unit,could it be harmonics?Possibly a worn tailshaft output bushing?
Im going to need a clutch some time this summer,so having any info I can get, will help when/if I tear the gearbox down.

Thanks.

 

I have the buzz in 4th gear on my 87 Turbo II. I also had it on my 87 NA that I used to have. It doesnt seem to hurt anything. I think its just because of old age. Ya know?

 

Yea,I figured as much since its not affecting trans operation the least bit.
Just need to decide on a full rebuild or a low milage replacement when the clutch goes in.
Im a nit-picker and everything (I do mean everything) on my car is new or upgraded, so the entire car works like it did on DAY 1......except that damn buzz!