Driveline vibration

[10PISTNZ]

My 01 GTS has a vibration in 6th at hwy speeds. It has been getting worse over the last year.
Is it possible it could be in the transmission?
Here is what I know so far;

- first off - it's not the tires. ( I switched tires and rims between my cars and vibration stayed
- If I drive 80MPH in 5th the car is fairly smooth. Very slight vibration that comes and goes with consistent frequency. If I depress the clutch at this speed and allow the motor to idle this mild vibration frequency doesn't change.
- However if I put it in 6th then it has a vibration that the frequency is more engine speed than tire speed.
- car has 355's
- it almost feels like a miss fire but no check engine light comes on
- motor mounts are new.

Could it be in the transmission? Any way to check?

[dave6666]

Check drive shaft and halfshafts for missing weights. Check brakes for loose or missing hardware. Check rear hub torque. Check u-joint clamps for tightness. Check diff mounts. Check diff fluid for contaminants. Check tranny mount and crossmember.

Those are the simple, easy and free ones. That should be done periodically anyway. Other items become more difficult or cost more than your time.

[10PISTNZ]

Thanks Dave

Some of those I checked, but not all. I'll check those next.
You're right about the time/cost

[uvbnbit]

did it do it before new motor mounts? factory or the poly ones?