Help w/ transmission Noise

[scotty-2-forty]

Preface: 96 240SX SE, KA24 @ 65000 miles. I recently swapped the OEM clutch/flywheel with a Clutch Master Stage III and JUN chromoly flywheel (in preparation for turbo build).

Problem: In fourth gear at exactly 45mph (2500rpm) there is a rumbling noise coming from the drivetrain. It is audible on deceleration, when lightly lifting foot from accelerator pedal. At first I thought it was coming from the rear differential, but with the car on the lift I had a friend bring the car to speed and it is definitely coming from the transmission. Apparently the sound eminates down the driveshaft and can also be heard through the diff cover. Occassionaly it can be heard in third gear, but not as obtrusive as fourth.

My thoughts: Could this be due to the clutch/flywheel swap? My friend who is an ASE certified Mitsubishi transmission specialist said that the swap and fourth gear being a 1:1 ratio (true??) could be resulting in a harmonic resonation. However, my experiences with harmonics have been a constant rumble which progresses with the vehicle's speed. This problem is only at the stated MPH/RPM in fourth gear and is not progressive. I even had to remove my B&M short shifter due to the noise being so apparent (AAK ... stock shifter again).

Your thoughts are appreciated!!!

[scotty-2-forty]

I am noticing too that upon cold startup and driving up to speed that the transmission noise is not as audible at as when warmed up. Also when cold, the car bounces forward as if the rear diff needed shimming (similar feeling to warped drum brakes); again, this lessens as the drivetrain warms. Thoughts??

[drjohn]

Could be a countershaft bearing starting to go out or possibly a chipped gear tooth on 4th gear. Hard to tell with out hearing it.