Nothing is as quiet as a 201 (possibly a Wheeler and Wilson No 9) but I don't think the bobbin case on the 401 should be the noise part, I thought it would be the swing arm and cam function? The solid cast iron absorbs more vibrations than aluminium. I wouldn't give up on the 401, and when you have one good and one half good at least you can compare. Is the bobbin case on the 401 and the 503 similar enough?