2000 Grandprix shifting problem

I have a 2000 Grandprix and occasionally it will shift really hard. I have tried a transmission addititive to attempt to lubricate the internal shift valves in the transmission, and the problem went away for a spell but has now returned. It doesn't do it all the time and there doesn't seem to be any consistency to when it starts shifting hard. One thing I have noticed is when it does start shifting hard there seems to be a "humming" coming from the drivers side of the engine compartment that is responsive to acceleration and RPM's. I did check the symptom while the car was in park and rev'ing the engine and the humming is still present. Anyone have any ideas as to what could be causing this problem?

Thanks for your ideas and assistance.

If you get it scanned with a trans capable scanner a DTC error code may show in the form of P18XX. The hard shifting and noisy sound like humming or whining is due to a fault commanding high line pressure. Typically a pressure control solenoid (PCS) should resolve the problem. It is a common and known problem on the 4T65E transmission. However, it is located with the valve body and requires that the side cover be removed for replacement. Not exactly a DIY job.