I recently had to repair AC on a 2001 GMC Sierra with 5.3. It had the oring leak out at the joint where the orifice tube goes. I could tell by the oily schmear on the passenger inner fender.
Check to see that your condenser isn't filled with bugs and leaves. Make sure your fan clutch is working properly and pulls good air at idle. Besides that, put some gauges on it and see what your pressures are.
I replaced the oring, orifice tube, hi/lo valve fittings, and accumulator. I also added a few ounces of oil to replace what would've been in the accumulator. I then evac'd and charged in just a hair shy of 3 cans. All icy now. The first time I charged it the hi side fitting leaked badly when I disconnected the gauges. So word to the wise... replace those fittings before evac/charging, or you'll be doing it twice.
Compressor knocking? Are you sure it isn't a bad clutch bearing or ac belt tension pulley? Either one is about a $20-30 part and some frustration. The compressor is much more than that.