Rear end hum in 5th only around 3200 RPM

[nastyleak]

Noticed the sound yesterday it happens only in 5th gear and it starts around 3100-then goes away at about 3300 RPM's. The car has unsprung ACT street disc and lightened flywheel which means I have expected drive train noise. This sounds more specifically related to the rear end. I have a recently rebuilt transmission from Jacks without a welded differential. Going to check the fluid level today just to make sure its not low.

 

[XiKeiyaZI]

When is the last time you've serviced the Rear end?

 

[nastyleak]

Only about 1300 miles ago, although that was 7 months ago. The rear diff was full and showed no sign of leaks at the time. I really want this to be a vibration but it seems different from the vibrations which take place in all gears due to the clutch.

 

[XiKeiyaZI]

Checked your carrier bearings and rear bushings?

 

[nastyleak]

The car has 112,000 miles and only recently has even returned to stock hp level (fixed boost leaks etc). I haven't ever even launched it yet so it shouldn't be broken from abuse.

I know it needs bushings and haven't checked the carrier bearings yet. If either of those cause noises or vibrations would it really be so pronounced at that particular situation? There is a lot of vibration from around 1900 to 2200 in 3rd through 5th but it is very different from this 5th noise and I believe the low RPM vibration is from the unsprung clutch disc.