1997 Nissan Sentra

I have seen a seized air conditioner compressor shred a belt. If that happened in the Sentra, you would not only be hot but stranded, since the same belt runs the alternator (well, you could still travel a short distance). A leak doesn't necessarily cause a compressor to seize, but I'd want to track down the cause of the squeak.

It may be coming from the belt or the bearing of any of the pulleys it runs: the alternator, the compressor, the compressor clutch or the tensioner. First, make sure the belt isn't loose. Push on it to see how much deflection it has. There should be only a little. If it's tight enough, next check to make sure it isn't worn or misaligned. Try squirting some water on the belt while it's running and squeaking (take care to stay clear of the belt and other moving parts). If the noise stops, the belt may need replacement or pulleys checked for alignment. If it doesn't, one of the pulley bearings is bad. You can try (with the car off) wiggling them to check for any play, or, while it's running, try to listen closely (again, watch out for moving parts) to see if you can tell more exactly where it's coming from.