I had good luck with some cheap updates to my air filter/intake. My 20 gallon belt drive compressor used a typical Campbell Hausfeld pump. The original air filter was lost in a move. My temporary solution was basically a straight pipe with a lawn mower filter. It was as loud as no filter. Inspired by my California Air Tools small compressor I bought an "automotive style" air filter and housing. The kind that looks like a plan with a snorkel sticking out. I slipped about 1.5’ of rubber hose onto the end of the snorkel. Its a very effective, low cost setup. That rubber hose really does help. It's a trick California Air Tools does. I wouldn't believe it had I not seen it.