I have a 94 manuel pathy.. when I accelerate going from 1st to 2nd gear I hear a rattling sound but as I pick up speed it goes away.... could it be the cat converter???

Most probably a heat shield. Is it at a specific RPM or specific to just between 1st and second? Manual or auto?

it's manual... and I noticed it only rattels when its cold and only in 1st and 2nd gear as i rev up and get into 3&4th gear it goes away.