GM-GMC: 2000 GMC Jimmy 4.3, transmission fluid change, gmc dealer

Question
My wife Jimmy has 122,000 miles on it.  It is running sluggish and ocassionaly "whistles" after the iginition is turned off and the engine stops running.  I know it needs plugs and wires, but the "whistling" sound has me stumped. Recently the transmission shifts hard on the 1-2 gear change, does it infrequently.  We had the transmission fluid pumped out and replaced and it hasn't shifted hard since, do you think the dirty fluid was the problem?  Thanks for your help, I really appreciate it.  I asked a local GMC dealer about my problems and he said those were $72.50 an hour questions.  What a guy! Sincerely, Bob  

Answer
THe whistling sound can be caused by an air conditioning system, after shutdown, as the Freon pressures equalize between the high and low pressure side.  Some vehicles are louder than others.

It may also be caused by a dirty air filter.  Have you checked the one on this vehicle lately?

The transmission fluid change also included a filter change inside the transmission.  This is probably what was was causing the hard shifts and they should not return.  Remember to have the transmission pan removed and the filter replaced every 60,000 miles (12,000 miles if towing a trailer) to prevent problems.