I've been getting a rattling sound from my exhaust recently. I have a budget Y-pipe, Random Technology Hi-flow cat. and a Greedy SP-2 exhaust. When I go from a complete stop I get this rattling sound from the center of the set up at low RPMs and it goes away at higher speeds. (I have and automatic if that matters for helping purposes).

I took off the cat completely to see if there appeared to be anything wrong on the inside but I couldn't detect anything (can't see too much just the meshy stuff)

Anyone else have this problem, or know what would be rattling from the inside of a cat.? (And its not hitting heatshields or anything like that)

Also where could I get a good cheap test pipe with the hole for the cat 0-2 sensor if I do have to end up replacing it. (just got it inspected so I'm good for another 2 yrs)

Thanks for help in advance.
-Steve