I haven't had a chance to jack my car up and check it out fully but my car has developed a rattle and i think its a blown cat... reason being we smelt a rotten egg smell the other day then the rattle started.. the smell has gone .. rattle is still there

Does the cat develop a rattle when blown? Might just be my heatshields but I'm picking its not.

 

I would start with removing the heat shield. Yes, some cats do make an internal rattle when they go bad. Cats normally don't go bad unless its fully rusted through, there is normally another cause for a Cat to go bad.

 

I also have an annoying rattle on from my exhaust. I think its coming from my muffler but im not sure anymore. It sounds like it is coming from the back of the car and my muffler was very rusty so I took off all the loose rusty metal and it still makes a rattling sound. But the sound isnt constant only at certain RPM with certain throttle input.

Maybe the muffler is rusted and it is rattling internally.

 

Mine does the same thing! My muffler rattles once it's warmed up at certain RPM ranges. I had just installed my Y-pipe, so I thought it was the cat, but no it's the muffler. Then I thought it was rattling against my hitch. It's hard to localize since it only makes noise when it's hot and running.

 

I'll be semi-pleased if it is one of the cats on the Y pipe - gives me the ultimate excuse to get a aftermarket one

 

Your cat can also go bad if you bump it on the pavement on a hard bump or something hits it real hard at high speeds...You can tell if it's the car by revving the engine up to like 5K and it should make a rather -sharp- rattle sound within the 3-5K rpm range

 

Yeah I've tried gunning it a bit .. think intake and engine drowns out the sound ... doesn't happen when stopped though.. but at a low speed roll it sounds like beans rattling in an old can

 

You should hear the rattling sound even when you're stopped and you rev the engine past 4K rpm...that's what I've seen in a couple of cars here at the dealership when their cats go bad

 

My exhaust rattled when I was Non-resonated. Maybe it has something to do with lack of sound suppressors.

 

If you have aftermarket exhaust or an aftermarker resonator on your exhaust, check that out first. I had the same type of rattle at certain rpms, once I replaced the resonator the rattle was gone.

 

I have a WS Ypipe and stock everything else. My muffler rattles when I go over a bump and sometimes at idle. When I push up on it with my hand it has about 1/2 inch before it hits something (my aftermarket hitch). I put the hitch on a few months before the Ypipe, and the rattle started a couple weeks after I put on the Ypipe. It sounds like the other guy said, a bunch of beans rattling in a can.