99.5 Jetta Interior Noise

If the door panel has never been removed then it could possibly be a bad speaker.

Being a 99 model it could also have a plastic liner under neath the door panel that has dry rotted and needs to be removed.

I'd place my wager on the speaker though.

If the panel has been removed check to make sure that you don't have any broken clips that attatch your door panel to the door.

maybe you could have a bad speaker or something loose around the speaker take it off and check it