Rattling under car when car starts or is cold.

We have reviewed and discussed your very short and very complicated question.
A bit more information would have helped but then again maybe this is all you had.

CLARIFICATION:
Is it a rattle or could this be a cracked manifold?

THE CLUE: "when car starts or is cold"
A cracked manifold has a very strange sound and may just sound like a tick or even a metal rattling noise BUT as soon as it warms it expands metal expands and closes the crack and the noise is gone.

RECOMENDATION:

Have your local mechanic inspect your exhaust manifold for a hair line crack and when its found you may see only a rusty spot where the crack is. It will most certainly get worse before it gets better if it is cracked.

Hope this recommendation guides you in a direction to solve your problem

R/
David

Check the Catalytic converter shield for a loose fit, ( it is on top of the cat. to shield the car from heat), or a broken weld at one end or the other. It is more like a rattle-buzz, if that is the problem.

You will want to go to an exhaust shop to have them look for broken welds or brackets as well as the catalytic converter. if it has come apart inside it will create this issue. common.