88 jetta engine rattle

I have noticed that my 88 jetta gl makes a loud rattling sound from the engine compartment when the engine hits exactly 3800-3900 RPM. I have checked that all the heat shields are in place and not loose, but I can't seem to figure out what is causing the noise... Could it be the valves? The engine is an 1.8 litre 8 Valve Digifant with "RV" code. I would appreciate any suggestions. thanks.