I drive an 87 non turbo. I recently re-did all the suspension, did a 5 lug swap, and lowered the car. Also in December I had to break down and buy a Bonez high flow cat to pass emissions for this last year. Now since about February I have noticed this rattle right underneath the shift area. It used to go away, but last night after driving through a horrible storm (probably coincidence) the noise was louder and present at a time I have never heard it. I called my dad who is an rx-7 guru and said if it sounds like rocks in a can I could just have rocks in the heat shield. I don't believe it is the drive shaft because I do not get that seat of the pants vibration like I did when my last drive shaft went. So this morning I get in it to head to work, starts up and idles like normal soon as I gave it a little gas it started rattling like a can full of rocks the entire time. So I turned around parked it and caught a ride to work. Now I need help figuring out what it could possibly be.

 

Have you checked for a loose heat shield, like your dad already suggested?

 

I don't have a way to get under the vehicle, my fiance has a jack and we will hopefully be doing that tonight, my concern is that my less than 6 month old high flow cat might be fried. It went from sounding like one rock rattling around at times to a whole coffee can of rocks rattling at all times.

 

No way to get under the car?..The car is EQUIPPED with a Jack!
anyways look for the Split air pipe from the cat to the engine hitting something or the Shields above the cat being loose.

 

Whatever you do, do NOT lift the car with the supplied jack then GET UNDER IT. Support the car with jack stands at the bare minimum before getting underneath it, especially if you are going to be running it to check where the noise is coming from.