I would put money on it being either the heat shield on the fire wall being loose or possibly some portion of your exhuast system. The noise your describing was identical to one encountered by a friend of mine. He has a 1993 Isuzu Trooper with the 3.2 DOHC V6.
Noise was caused by a faulty exhaust hanger and eventually the heat shield around the exhuast manifold/header area.
190E 2.6 - 244 000 km