2002 Buick Regal Unknown Tranny Problems, HELP!

I have a 2002 Buick Regal with 118,000 miles on it, it randomly started making a wierd noise, almost like a bad serpentine belt would make. It also shifts hard and bangs when I shift from overdrive into nuetral when coasting. It only does this when i do city driving, if I start the car and get on the highway, it does not do it. When the problem occurs, I can shut the car off and start it back up and it will be fine for a little while. After it started doing this, I changes my oil and filter and therewas no metal or anything abnormal in the oil. Im guessing its electonic, Please Help! Thanks!!

Is the noise more like a whining or grinding sound? I'd get it scanned with a trans capable scan tool to see if there are any DTCs stored like slippage or shift codes.

It doesnt make a grinding noise or a whining noise and I have not noticed it slipping at all.

You may not feel any slippage physically but that doesn't mean there is no internal slippage. The PCM module will determine that based on internal sensors within the autotransaxle. Not all tranny DTCs trigger a SES/CEL light.

The only other possibility that comes to my mind since it appears to be just a 4th gear - OD related problem is a possible bad 4th shaft which is common on a 4T65E autotransaxle. When the shaft gets real bad you will lose 4th altogether and it will bang and then seems like you are in neutral. The splines on the 4th shaft strip out due to the material being of poor quality or non-hardened.