1999 Other Isuzu Models Metal on metal rattle in D

'99 Isuzu Amigo - Getting a intermittent metal on metal rattle (sounds like a coat hanger on a metal fan blade) when the vehicle is in D and stopped. Can reproduce somewhat when engine is revved in P, and gets somewhat worse when A/C comp. engaged. Sound diminishes when car is in motion and accelerating. Ideas? Could this be a serp belt tensioner, a pulley going bad (not a squeak) or something in the exhaust? Perhaps the Catalytic converter?

Thanks!