Good guess, that's the Air injection pump.....it's not that. The one I am on about is much smaller and mounted further back on the inner wing and on the top of it. It fits on a small metal bracket and is mounted by a rubber bush. It also has a small pipe that joins to a "T" piece so would appear to be a pump of some sort. When the car is in "Park" with the engine running it does nothing but when you put the car in "Drive" (with the handbrake securley ON, so it doesn't run away) then you can feel it "pulsing".