I have noticed that during low RPM starts in first, I have a chattering and rattling sound from around the transmission area. This occurs if I am in first and start at under 1200 rpm. For example, moving slowly up my driveway. I haven't exactly gone under the car to look if it is a loose heat shield or whatever it may be. I haven't heard much rattling in other situations, so it may not be a loose shield. Would anyone like to give an assumption as to where the chattering is coming from?

 

Same issue still...

 

When I worked for a muffler shop, the "test" we did was to put the car on a lift ,or ramps, and give the cat a heavy whack with my hand, if it rattled, we would replace it. Oh, and wear gloves

 

Had an assumption that it would be the cat, which I had a suspicion that it is starting to fail.