So a week ago, I installed a Powercore header on my 99si.
Acceleration seemed improved (and the sound of the engine too) especially above the VTEC point.
Then I started to notice a rattle at around 4,000 RPM.
After trying all the usual stuff, (removing the heat shields, checking for cracks in the welding, re-torquing the cat bolts) it was still there.

It turned out to be the stupid bracket for the old exhaust manifold. I guess that without the heavy manifold bolted to the end of it, that bracket rattled itself lose from the engine block. I guess you can add this one to the list.