[f5fstop]

marin8703 is right, there is no oil pressure gage.
The loud noise you hear on engine startup is what they call the "dirty hose" for the PCV system and the vacuum in the engine is causing the loud rushing noise.
GM did a bulletin on this for the 3.5L engine, that tells the dealers/techs it is a normal condition for this engine.
Honestly, not exactly sure why it happens only upon startup. However, I did find out it might happen at other times too. It happened to me coming down the Lasal mountain loop road into Moab for about two hours, and drove me crazy, and then it went away.