When I first turn on my AC after my engine hasn't been running for a while, it makes a weird rumbling noise. It sounds like, "grrrunk grunk". It is two or three short bursts, then it doesn't do it anymore. It won't do it if I turn the AC off and back on. It only does it after the engine hasn't ran in a while. BTW, I did add refrigerant a couple of weeks ago. The pressure is around 45 psi. Any ideas?