1999 holden vt commodore rattles loudly from the engine once warmed up, but stops when revs increase, help?

i have a 1999 holden vt commodore, once the car idles until it is warm it begins to rattle from the engine, i had a listen with the bonnet open and thought it was the tentioner pulley bearing, but once i replaced the pulley the noise was still there. It is hard to tell where the sound is coming from, it sounds like it is either deep inside the engine or towards the back and bottom of the motor. I have read that it may be my exhaust manifold heat Shields, I am not 100% what they are, but i have looked at the manifold and cannot see any sort of shields around them. Does anyone have any idea what my problem is?