98 civic door panels

when i was sawing out the basket shelf to fit in better speakers i noticed that there was a bunch of white plastic stuck to the inside of the door panel. this stuff vibrates like hell - is it there for any reason in particular? i assume its for waterproofing; has anybody removed it/replaced it with something that doesnt cause as much noise? dynamat is a last resort considering its quite expensive, if theres an alternative id like to hear it..