Hey I have an 02 Accord and lately it's been making a weird rattling noise under my car, it's part of the muffler and it sound like there's chunks of rock in there or something, what could it be? It only sounds when I'm on idle or driving slowly.

 

take it to a muffler shop, then run the car when its on the lift and check for the noise. just tell them you think you have a leak or something and see if they can help you out.

 

if your someone mechanically inclined i suggest you take off your catalytic converter and inspect it to see if it's broken

 

You might need new cat, when broken, it like full of sands inside
making weird noise.