New to the forum, so I hope this is the correct way to post. Heard a slight hissing noise emitting from right side under glove box, then idle became rough and erratic. I turned off A/C and problem stopped. Could this be a bad bearing or clutch on the compressor? Problem only occurs when defrost/air conditioner selected.

 

Welcome! I'll move this to General Tech where the actual questions belong. This place is pretty much just for welcome & introduction stuff. You did everything else right - LOTS of people don't even tell us what year car they're talking about.

Open the hood & set the AC so it's making the noise. Try to pinpoint the location. Seems like compressor bearings would make a rattling or grinding "bad bearing" sound not "hissing". And the sound would be located at the compressor.

It might be the expansion valve is plugged? Maybe incorrect amount of refrigerant charge? For either of these, you should get the system pressures measured. You want to measure both high and low side pressures to diagnose inventory or plugged valves.

Beyond that, there's a couple other regulars on this board who know AC systems better than I do.

 

I agree w/ Jim. Sounds like AC compressor is loading up, perhaps a blocked expansion valve or damaged expansion valve. Don't run until you're ready to measure pressures.

good luck