not sure if it's been mentioned yet, but pump speed has a lot to do with noise too.
case in point,
I own a Campbell Hausfield compressor, horizontal tank I think 30 gallon, from the early 90's , when HP was severely over rated . I think it's listed as 4HP
My buddy has the same era vertical compressor, probably 60 gallon, rated at 6 HP I think.
we once compared the pumps & motors, physically look the same, his is higher amp draw and larger pulley on the pump. also higher CFM . this is only because of the higher rpm, Im sure
his compressor is way louder than mine.
I suspect if we swapped pulleys , we would have results that match .