My 2-yr old 12,000 BTU Friedrich wall AC unit started making a very loud buzzing noise recently. The noise does not occur in the "fan-only" mode, but starts about 1 minute after the AC is turned on. The noise is louder inside the house than outside the house, and seems more "electrical" in nature than vibratory. As far as I can tell, the cold air output is still OK, despite the noise, but I'm not going to use the unit again until I find out what the problem is. Thanks.

Odds are the contactor which engages power to the compressor is going bad. It will "vibrate" as it tries to make/keep contact and make a buzzing noise. Have a tech. check it out. Not a major problem, but it needs fixing...