97 Bonny SE, 75k. Electronic controls. The AC blows cold, When the AC is on, there is an audible hum, that seems to be coming from the blower, it seems like the blower speeds up (but doesn't push more air) and the pitch is higher with acceleration and vice versa. When the AC is off, there is no noise at all. Any ideas?

Are you sure the sound isn't coming from your vehicle speakers?

Here is a post with similar issue that another member posted recently. I am not saying that this is your problem, but it kinda sounds similar.

http://www.bonnevilleclub.com/forum/...ic.php?t=83588

No, it'* not from the speakers. It goes away when I push the AC button. The car is car is quiet, then push the AC button and the blower hum comes back.

Hmmmm.......not really sure what it is. I wonder if the compressor clutch is humming when engaged. I really don't know much about this, but maybe you should pop the hood in park, turn the A/C on, and see if you can hear anything in the engine compartment near the compressor or blower motor to better help isolate the location of the noise.

Try unplugging the blower motor when it'* happening, and see if the noise stops.

The compressor does have a different sound when the AC is off or on, not the usual sound a compressor should make. With the hood up, I get a whine/squeal from the compressor area when the AC is OFF. It goes away and sounds normal when the AC is turned on.

How do I unplug the blower motor, not sure where the plug is, engine compartment?

On the firewall behind the engine is the blower motor. There is only one plug.

It appears that your clutch bearing on the compressor is bad. As for humming... could the sound be the fans turning on with the AC? Your cooling fans are programmed to come on anything the AC button is pressed.

OK, if it is the clutch bearings, does that mean a new compressor? Can the average backyard mechanic change it out or do I need an AC shop? If it can be changed at home, any instructions or links? I have the Chiltons for the car.
Thanks

You can change just a clutch. My understanding is it takes special tools etc, but have never seen one done or done one myself.

A mechanic buddy of mine put it this way. "If you have to remove the compressor to change the clutch, save the time and put in a new compressor." Part of his reasoning is that he'* not the one picking up the cost of the compressor vs. clutch. However an extra hour of labor to swap the clutch comes close to the cost of the compressor.