I just did that i hit the y pipe with my hand and it made a rattle noise when i start the car cold it dosnt make a noise it usally starts when it warms up. Should i take it to a mechanic or a muffler shop.

 

If my cats gone bad should there be ash coming out of the exhaust also do they make rattle noises when they dont work no more.

 

Interesting that you should mention that, when I had the O2 sensor rattle it also did not start till the car was warm also.

 

The ash could be the cats breaking apart, and they would rattle if pieces were loose inside the cat.

At this point, it could be either one (O2 or bad cat) making the noise.

Next thing to try would be to pull the rear O2s and plug the holes and see if the noise goes away. There is a certain size oil drain plug they sell at AutoZone that will fill that O2 sensor hole, that's what I used but I can't remember the size. 18mm maybe? Maybe someone else can confirm that size??????

 

I really hope it's exhaust related because it would suck if i spun a rod bearing. This rattle happend like 2 weeks ago and went away but came back.

 

whats the exhaust rattling when you hit it have to do with a rod bearing? you would have low oil pressure, etc. take their advise and use a rubber mallet, hit the cat, does it rattle? hit around the o2s, does it rattle there? you should be able to isolate the source. its most likely one of those.