Hum sound underneath when recording

That's probably the last thing you need to be doing... When I said grounding arrangements, that's what I meant - often these things happen because there is multiple grounding occuring when there shouldn't be - that can cause a ground loop, which is what generates the hum. But it's not even guaranteed to be this; there are other possibilities as well, especially with low-harmonic hums - just the positioning of a mains transformer can do it sometimes, especially if there are unbalanced connections in your system. Really, you should get somebody who understands about these grounding issues to look at it, as altering grounding arrangements can potentially create conditions that are more dangerous than they should be.

There are a few things you can try, though. First thing to do is remove the mic and compressor completely and see if that removes the hum when you record. Then plug the mic directly into the mixer and see if that's okay. Then if you plug the compressor in again, and the hum restarts then you'll know that it'a a ground loop between the compressor mains and your computer. That, FWIW, looks like the most likely cause of your problem, because it's the only other externally grounded component in your chain. But because it's not in front of me, I can't be absolutely sure - it may be something else completely.