car doesn't sound good......

my car is making a rattling sound pretty much for every mile i travel the rattle is only for a 2 seconds an stops . its under the car and sounds like the drivers side. i just had the catalitic converters sealed up cause they were blown out. i thought that it might be a bolt in there in the exhaust. the only thing is it would rattle constantly i think insted of 2 seconds after every mile. anyone have any ideas what it could be?