I have a 2000 RX 300 which I just had my mechanic replace the alternator, belts, etc. For the first few days there were no problems, but since the belt has begun to start squeeking when the A/C is on, and sometime seems to get worse until I turn the A/C off. Could this be simply a loose belt, or does it sound like the A/C Compressor could be going out? I wasn't sure if these have a self-adjusting tensioner, or if it was something that I could possibly work on. Any suggestions???

I Had the same with my 99 Rx300. check your alternator belt, that was the problem with mine. There are 2 bolts you have to adjust: first loosen the bolt left of the alternator, then adjust tensioner.