What the heck is up with my AC? Weird sound, car jerking...

[BoyYota]

Hey guys, so when I got my car the AC seemed fine, but eventually I realized that it actually hardly ever blew cold. I took it to one of my mechanics to have it refilled, and he said he didn't see the pressure gauge show any difference so he thought it all leaked out. But it worked fine for a few months. Then we had our first hard rain. After it rained, ever since I turn on the AC, I hear weird sounds that seem to be coming from both up front and the rear of the car, at first I thought it sounded like little bubbles, but all it's doing now is making a 'WHOOMP' kinda sound, not a squeal or grind, but something softer than a grind, kind of in between. It is often accompanied by a jerk of the car. It seems to be doing it more and more, and more often the compressor will just seem to fail to engage after a while. What do you guys think, is the AC clutch failing? I haven't tried handy old WD40 yet. The only mechanic I told about it so far said it's normal for the heater core to make weird sounds like that... perhaps I should have told him how it felt a bit more serious than just a little sound from time to time.