I'm not sure that there is any on the market that do not 'hummm', since that's a byproduct of how the relay works... it's magnetic and requires power to keep it in the 'switched' mode, which is why you hear a hum. It's also a byproduct of where the ATS is located, which for many of us is right in the base of the bed, or close by.
Without having to pull the whole box, and rerun the wiring(a pain!), what if you purchased the same unit as a replacement, and only replaced the 'guts' of the unit(the relay switches)? Might be an easier overall job.
But, yes, I would move my ATS to an outside storage bay, though it can be a job handling those large wires, but maybe it's an idea!
'14 Palazzo 33.3 bunkhouse 34' diesel
KingTailgater2 Dish HD, 100w/5a SOLAR, BlueOx
100,000+mi since '14 - US, Alaska, Canada.... formerly Georgia, then NorthCarolina, now Alabama : )