rattleing noise comming from transmission

What kind of transmission, and what are the conditions when the noise occurs? Parked, driving, shifting? Give us some more info and we may be able to help, but the best thing to do is have a mechanic listen to the noise and get their advice.

If the transmission still "works" could the noise be a "bad" catalytic converter that has broken up internally and rattles, this noise often sounds like there are golf balls in the exhaust system.

Mine was sounding like a tank, ended up being the fuel pump going bad.