Been noticing the a/c compressor in my truck is making a loud "whirring" noise that increases in intensity with RPM. It seems to be louder the higher the heat load is. I'ts hard to describe the noise. It's not the clutch. A/C is working great otherwise, it is ICE cold. Think it needs a little oil?? Worn out? I thought it might be overchaged from the shady used car dealer I bought it from but I don't think it would be working as well as it is if that were the case.