grinding in second third and fourth

i have a 2000 jetta. when i am in second gear, and i accelerate, i hear this grinding/rattling sound. it has nothing to do with rpms because when i gas it in nuetral it doesn't do it. it also does that in third and in fourth it sort of pops the grinding. but it's only when i get the engine speed up and it's in those gears. i looked underneath and i noticed one of the plasitc pieces on the skid plate, where it bolts to the body, is busted. anyone know what it could be?