I first began to get a rattle noise from what I thought was the catalytic converter. It only rattles when I first start the car and then goes away after about two minutes. Then I started to have problems accelerating. I replaced the cat, but that did not fix the noise problem, and it seemed to run fine for a while. I think the noise may be coming from what I guess is a resonator.

Now I continue to get the service light with code P0101, indicating a error in the MAF. The rattling has seemed to stop, but now I can't accelerate past 30mph.

It is a 2002 2wd 2 door 70200 miles.

 

Same thing happened to me last year. Rattling noise under car and poor acceleration. The rattling noise was the element in my catalytic converter that had begun breaking apart and would partially plug the exhaust flow when I sped up and. I replaced converter and downstream oxygen sensor and problem was gone. I used a Walker direct fit catalytic converter from Advance Auto Parts which was substantially lower in cost than an OEM. Their part # for my 1996 Blazer is 15827. This cleared up the problem. No more rattling and normal acceleration.
I hope this helps.
Curly

Another possibility may be your EGR valve. I took mine off and cleaned it out. When I reinstalled it, I used an "Anti-Clog" type gasket to keep carbon from interfering with EGR valve operation.

 

The rattle was still present after I changed the cat. What is the part in between the y and the cat? Is it a resonator? Is it possible that the noise could be coming from within it? Are there any internal parts within it that could have come loose and are plugging the exhaust and restricting the flow?

 

The 2002+ exhaust system has a pre-cat that is directly behind the Y in the y-pipe. It is possible that the rattling is coming from this location. It could also be a broken heat shield.

I would clean your MAF sensor first to see if the P0101 code goes away. If not, check out the 2nd sticky in the Tech Article (DIY) section to see a summary of all posted trouble code sheets.

Or just click below:
P0101 - Mass Air Flow System Performance

 

I took it to a local shop and they confirmed with me that it was in fact the pre-cat that was plugged. They replaced the whole assembly for $800. Which included the y pipe, pre-cat and regular cat. Now it runs like it should. I would have done it myself, but I was scared to death about breaking the bolts free from the manifolds. Still can't understand why it even needs a pre-cat.

My thanks to this forum for the help and insight.

 

Glad you got it fixed. Thanks for the follow up!

 

I think I may have the same problem with my 99. When I start it makes a rattle sound and it stops when i put it in drive. But I was told its jsut a leek somewere but it also has problems getin up to speed.

 

I have a problem when in DRIVE it RATTLES LIKE CRAZY , then if I put it in Neutral it stops 99% of it.
I have been told its the catalytic converter , but I was hoping it was the Heat Shield , but I dont think I'm that Lucky

Is ther a dummy proof way to test if its the Heat shield or the Converter ?

99 Blazer 4WD