I have a 1998 Isuzu Rodeo 4 Cylinder, 5 Speed, 2 wd (The base of all base models) and when I start moving there is a vibrating sound coming from under the vehicle. It sounds like a vibration sound. The noise continues until I am fully off the clutch, and happens again while shifting into 2nd gear and 3rd. I also notice it at idle when I first start up, and if the rpms are pushed near 4k. I had a mechanic look at it and he thought the noise was a resonating noise coming from a pipe in the muffler that contains the catalytic converter and he said to replace the entire piece would cost over 900 dollars. I am not so sure I trust his judgement, so I have lived with the noise. I read the threads about the idle pulley but I can't tell if this is the same noise. About a week ago I saw a Ford Expedition making the same noise. Any help anyone can offer would be appreciated. Thanks.

I have a 1998 Isuzu Rodeo 4 Cylinder, 5 Speed, 2 wd (The base of all base models) and when I start moving there is a vibrating sound coming from under the vehicle. It sounds like a vibration sound. The noise continues until I am fully off the clutch, and happens again while shifting into 2nd gear and 3rd. I also notice it at idle when I first start up, and if the rpms are pushed near 4k. I had a mechanic look at it and he thought the noise was a resonating noise coming from a pipe in the muffler that contains the catalytic converter and he said to replace the entire piece would cost over 900 dollars. I am not so sure I trust his judgement, so I have lived with the noise. I read the threads about the idle pulley but I can't tell if this is the same noise. About a week ago I saw a Ford Expedition making the same noise. Any help anyone can offer would be appreciated. Thanks.

It could very easily be something loose along the exhaust lines. I would get under there and check to make sure everything is tight. I had something similar happen in my 3.2L once and it turned out to be the catalytic converter heat shield rattling. Mine is an automatic....but it sounds kinda like the same deal...it would make the noise upon initial acceleration and stop after I got up to speed.