New (to me) 2004 S10 4x4 4.3 Vortec
 

It's in nice shape but the usual used-car issues popped up. I replaced the downstream O2 sensor (no biggie) but now the AC compressor is making noise. AC itself works fine (plenty cold) so I assume the noise isn't due to low refrigerant

Sound like the compressor itself, yes?

First, compressor not engaged
http://www.ah6le.net/not_noisy.mp4

2nd, compressor engaged

http://www.ah6le.net/noisy.mp4