... sounds an awful lot like the exhaust gasket leak on my dad's tahoe... then I thought about it a bit more. When I was reinstalling the valve cover, I snapped the lead bolt holding the cover down (at the front of the M30, where the timing chain is)... Ya think that could cause the noise??

Not sure how I'm gonna get that bolt out, gotta find a skinny EZ-Out or something... I bet that could be it tho.