Looking for help on a intermittent hum/noise from new tube mic. Noise floor jumps about 3db or more occasionally. For instance - while recording vocals - punching in/out... inching my way along through verses... I can solo the track after the fact and occasionally the hum will be there - 1st vs. its there, 2nd gone, 3rd back again. Settings are not changing during the course of recording. I thought it was the compressor on my fridge kicking in, but I've rulled that out. I do live in a row home with common walls to other houses and I'm suspecting I may be getting the AC from a neighbor. ??

Suggestions on where to start?

Is there any kind of power conditioner that would keep my power clean - budget priced of course?