I wonder if it isn't chain noise? You stated you just tightened things up. Try a bit of lube on the chain to see if the noise goes away. How many miles on the chain?
Is the noise intensity in sync with road speed or with engine rpms?
Just reread your post and from your description the noise appears to be linked to high engine rpms. Only thing that comes to mind is the cam chain tensioner but that is only a guess.
