Hello,
Strange thing going on. When I fire my truck on I get this LOUD resonating, vibration sound coming from the muffler area it seems. There is a thin sheet of metal that goes over the top of the muffler (I am guessing a heat shield) When I put my hand on it dampens the sound. It is getting louder and louder with time.<O</O

It only does this at start up----as I rev the engine the sound revs with it. Any ideas? Clogged cat converter, muffler, loose parts? <O</O
<O</O
******Also, I am looking for a .PDF version of a repair/service manual for my 2005 Chevy 2500HD. Does anyone know where I can find one? I have checked the torrent sites with no luck.
<O</O
<O</O
Any help is appreciated.<O</O

 

well one problem to consider is a bad cat. i have an 04 1500 and it went out at 87k. it made like a rattle noise at start up as well and with acceleration as well. it was embarrassing to drive with that noise. and the cats arent cheap since aftermarket isnt an option.

 

i agree with s34n. ive had a cat do that too. be careful. if the cat comes a part completely it can stop up your exhaust.

 

Thanks, tonight I will post a link to a vid showing the noise it makes....

 

Thanks to Alphachevy for posting this query and the others for the answer. My Blazer started with a rattle/resonance just like this. Have just checked the cat and yes it is this rattling. Looks like my powerflow exhaust system might be needed earlier than expected. Been told that with my LPG conversion a cat might not be needed. But what happens with the O2 sensors?
Adrian

 

without the cat the o2 sensor behind it will misread and cause your check light to come on. since i cut my cat out, on average, my fuel mileage dropped between 1mpg to 1.5mpg and i have to pull out the 2 fuses to my ecm out for 15 minutes to reset the computer every 2 1/2 weeks. pulling the fuses out sure beats disconnecting the battery. one thing u can do about that o2 sensor is go out and get a tuner program and turn the o2 sensor off. im not going to use the tuner on mine until i put in the 98 350 engine and 98 ecm in mine. since yours is lpg i dont know how much of this will help.