Need some help (or not) with what sounds like a 50's hot rod noise coming out of my EGR/Air injection pump system. When I unplug the vacuum line on the outer side of the "canister" the noise stops. It makes the noise louder as you rev the engine. I have no exhaust leaks (been inspected). My car is also throwing the PO420 code. Is this related? Is this a problem with EGR? Is there an easy fix? Is there another post dealing with this? Thanks.