The biggest culprit is plugging your modular synth into one outlet and having your recording system (computer and audio interface) plugged into a different outlet. If you can get both parts plugged into the same outlet that might be helpful for reducing or eliminating that ground hum. That would be the first thing I'd try to fix.
-- Ronin1973
Spot-on, Ronin. I've found that the majority of audio issues revolve around that problem. The fix in a large studio is to star-ground EVERYTHING...including the AC outlet ground pins! And they'll use some voodoo wiring for that, some OFC or Mogami alloy nonsense...but really, all you'd need is some stranded 18ga hookup wire from Home Depot, etc and you don't necessarily have to go bonkers with wiring every...single...outlet. Just connect everything so the wires aim toward their destination at the groundpoint on the mixer, and that SHOULD fix at least 80-90% of the crud. It's also a good bit easier than diving into the modular's guts...but if you've gotta, you gotta!