Did it sound like a roll of firecrackers going off in a parking garage?
Probably just the turbo anti-lag system kicking in.
---------- Post added at 10:00 PM ---------- Previous post was at 09:53 PM ----------
Yes, valve float can make it sound different.
So can an overly lean combustion, which could possibly result from spinning an engine at 5000 rpm without jetting the carburetor.
Or it could be trying to stuff too much exhaust through a crappy stock muffler, or suck too much air through a stock air box that doesn't breathe, or both.
Or you could have a loose bit somewhere that resonates at 83hz and adds a funny sound when the engine hits 5000rpm.
Or there could be a squirrel stuck next to the flywheel.
Without hearing it, it's kind of hard to diagnose.
But yes, it could possibly be the result of the valves starting to float.