Originally Posted by sgp358
One common issue with the 4T65E is stripped splines on the 4th clutch hub. You could be intermittantly losing connection between the 4th hub and the overunning clutch. Are you sure it'* a 4th gear problem and not a TCC issue as Dan has pointed out?
This is a common problem that GM fixed in 03 by using a shaft with hardened splines.
When my shaft went out, it would sometimes whine when trying to go into fourth.
Also when you lose 3/4 as opposed to TCC/no TCC there would be a more drastic change in rpm.
The trans shop confirmed my case by finding some fine metal shavings in the fluid.
They checked this somehow without removing the pan.
If I were to do it, I would put some magnets on the outside/bottom of pan, drive it for a while, then drop pan and look inside near the magnets for accumulation.