Check the pressure blow off valve, it's probably on the top of the air storage tank. They stick open, especially with a little crud and corrosion in them. Fire up the compressor, and pull the manual operating handle a few times to let air pressure flush the crap out. This usually fixes them, for a while.
I removed my engine driven compressor (air horns and shop air) and replaced it with a 110v 1/2hp compresssor from Sears. Now I have air without running the port engine. It makes no sense to have shop air in the engine room, but not be able to use it on one of the engines unless it's running.
My boat already had the air plumbed to several regulated outlets in the engine room. Very handy.
I plumbed the compressor's output (after the regulator) to the existing boat air tank. That way I have twice the storage capacity, with the compressor's tank, and the boat's tank. The compressor fit easily outboard the port engine, under the exuast hose, in a place it was hard to walk through anyway.
The compressor cost $139, the custom made braided ss hose to hook it to the boat cost $100! Oh well, at least it's done right.
I ran a dedicated circuit from a spare breaker in one of the ac panels to power it, as the circuit nearby already had a cockpit freezer and a bait tank pump hooked up to it.