It happens on cold start, mostly when I go home from work. Parking in the garage at home, it doesn't do it or at least not nearly as much. Goes away mostly when the car warms up.

It doesn't sound like the engine, its louder further back towards the trans, but it's hard to tell. The most annoying part is, by the time I get home to try to see what it is and get it up on stands, it goes away.

 

Does it go away if you push in the clutch?

 

It does not

 

Something loose rattling it sounds like to me. Probably an exhaust heat shield, or something on the exhaust system making contact with something it shouldn't (car body, transmission, rear subframe, etc).

For my car I made heat shields on the exhaust system and they rattled. I ended up giving them all 1/4" clearance minimum as if I didn't, they would rattle when in decel, or when accelerating, or at 1,500-1900 RPMs. Weird cases, but giving them a bit of room to shake without hitting anything fixed it. If you got a spot where it's close to something I would start there.

 

Thanks guys. You may be right. I'll probably just have to get it back in the air this weekend.

 

Just in case anyone else checks, patsmx5 was close - it's crap IN the muffler. I don't know if it's from the old cat that died or what, but when you hit it, it sounds like a maraca.