I have a 7hp four cylinder comp that was annoying me. I pit mine in a corner and then bought some noise absorbing foam from MSC with sticky back and stuck to the two walls bahind it. Also built a small frame screwed into the corner to hold two sheets of extruded foam on the other two sides. It helped quite abit but it could be better.
The neatest thing I saw at one company was they had built a little room for their comps with concrete blocks and filled the holes in the blocks with sand to absorb sound. You could actually here yourself think in that place. Plus they had storage up on top of it then.I think they had pipes run outside to for the inlet and blowoff lines.