Balancing/Quieting Air Compressor Piston

Hey all,

Old air compressor (was setup to be used with a paint sprayer) runs at about 75db. I pulled the cover off since it was rattling some, but no true reduction of noise. I’ve narrowed it down to the compressor piston itself (and the shaking it produces). Turning the thing by hand I can tell the counterweight weighs more than the piston, so it’s unbalanced. But, when the piston is actually compressing the air pressure is effectively making it heavier (more inertia)?? So I shouldn’t balance it while stationary?

Any suggestions to quiet it, of just put up with it?