a couple of weeks ago my 2004 stx 2WD started making a rattling sound. (at first i thought it was a rod knock and was like oh **** because i just pulled and refreshed the whole engine a couple months ago! lol) but now i'm 99% confident it's the exhaust.

it only rattles after after the truck gets to normal operation temp. also it only rattles on acceleration or under load, except for wot when it goes silent but then it starts rattling again after driving normally.

nothing is loose under there. all 4 cats have the strapped heat shields intact and tight.

so i'm thinking it's gotta be internal to one of the cats (sounds most likely like the driver's side rear cat from what i can tell).

but if the honeycomb material inside is rattling around, wouldn't i get a check engine light or code? also is there any other way to check other than pulling the exhaust?

gas mileage is slightly down about 2 mpg since the noise started. otherwise, the truck runs fine.

thanks

 

The PCM only monitors the converters to see if they are working well enough to maintain compliance with emissions regulations. As long as they do that job well enough, the PCM stays quiet.

Suggest getting under there with a rubber mallet or dead blow hammer and see if you can induce the noise then follow it to its source.