Hey everyone. My 1992 w124 300E has developed an odd knocking noise that I could use some help figuring out. The noise is not apparent when the car is first turned on (cold), but as soon as the air pump turns off (as gauged by the lack of a rushing air sound) it appears. It sounds like somebody is hitting a flat piece of metal with a small hammer and increases in frequency with engine speed. It is loud enough to be heard in the cabin with the hood shut and the AC on. I am inclined to believe it is the air pump but I can't explain why or how. The engine has been idling a little rough when cold but otherwise has been running quite well. Any ideas? Thanks.