Started the car after lunch and there was a semi hi pitch hum. went away about the same time the air pump stopped.

It would only hum at idle, its an M6, 2002, 26k miles.

Any ideas? I have had intermittent codes suggesting air pump/check valve/rear 02 malfunctions. All of which clear themselves pretty quick.

 

Mine does the same thing...and I have virtually no mods so I know it's not something I did. Also I don't have any engine codes popping up but the noise you describe sounds just like mine. I'd also like to know what the hell it is.

 

That would be a chack valve. I'm 95% sure it is. I had the EXACT same symptoms. Got the same code, except I didn't get the O2 sensor code. But a bad valve could cause it to throw a code. Check both valves. Sometimes the code gives the wrong bank because it can't tell the difference. I changed the valve and the code went away after three or so starts. If the valve is bad, it should either let very little or no air through it, or it might be letting air through the wrong way. Whichever one is worse, replace it.

Good luck!