When recently replacing my LIM gaskets (2-3weeks ago) I cracked the upper plenum and a couple days ago I noticed a faint knock from under the hood when starting it up that would slowly go away after a couple minutes, didn't sound too bad so I didn't pay much attention. Today I started it and it was at least twice as loud and didnt go away. Is it obvious that the cause for this knocking is the cracked plenum or what else could it be? Any suggestions or oppinions would be really helpful