I have a 1998 Ford Ranger XLT v6 5 spd 4x4. Everything works pretty good but 1 annoyance. When I run the air everything works good for awhile but sometimes when I come to a stop I hear a very loud HUMMM noise coming from the front of the truck. When I start driving it goes away but when I come to almost a complete stop I hear the noise start back up again. This only happens when I run the AC. Does this mean my compressor is bad / going bad? Or does it mean that it may be running low on freon? Or something else? Any help would be greatly appreciated. I love my ranger and want to keep it for a very long time (91000) miles and paid off! Trying to make this thing last as long as it can.