It sounds like one of the preamp tubes might be the culprit. A pair of 12AX7s I think? You might try swapping them out first. If it solves it then you're all good, if not then you can look a little deeper. You could buy one (or two - doesn' hurt to have spares) and try it in each position, to see if one or both are at fault.
Ok so it's in your recording chain, it could be a bad mic cable - good first place to look. Try another just to eliminate it. It could be a ground loop or even bad grounding in your home that could be the problem. Have you tried recording with your Mac in a different location? Seems strange the SM7B would be quieter, but then it does need a hefty amount of gain in the first place
Cheers