Anybody Know Anything About Air Compressors?

It might be...

Compressors (mine at least) doesn't have valves in it like a car engine. It has a couple of metal flap valves that swing on a couple of metal pins. They are sucked open and blown closed by the air movement. Sadly I don't have photos.
One of them can tap the top of the piston when the pressure in the receiver is high. It sounds to me like a metallic rattle on my compressor. Almost like loud pinking on on a petrol engine.

If mine is achieving pressure I don't worry about the noise. But if the head gasket is leaking it'll rattle more and not achieve pressure. Or if the valve is bouncing (I think on mine the head gasket puts some pressure on the flap valve to bias it shut) it'll struggle to get pressure. At that point I take the head off, cut a new gasket from gasket paper and turn over the flap while I'm in there.

They do run hot by the way. Compressed air gets hot. That's why turbos have intercoolers. It's also why you need to drain the tank each night. Hot air carries water which condenses in the tank and can rust it out from the inside.

James

I rebuilt my compressor pump last year, the conrod bearings were starting to seize so i emery papered them and cleaned everyone up. Now run it on 5w30 motor oil. Smells lovely when the compressor warms up!