How to address loud cold air return
I'd have to say get an HVAC guy or firm. My first thought is that they could relocate this one under the stairs &amp; have it suck through the side wall of the stair's stud bay. My 2nd thought is to add the stair stud wall as a 2nd smaller return, but then also move &amp; shrink the floor one away from the stairs if there's another 4 or 6' away it could be.

The idea is that there should be a large sound reduction by getting that return out of the direct line of the HVAC fan's draw. You'd be getting rid of the direct sound path of the fan motor &amp; fan, but also adding distance between you &amp; the fan noise &amp; splitting the suction whoosh in half. Definitely, see what they say. I don't see any of the other options doing anything, just like what you've tried &amp; determined already.