Yes, that sheet is for slowing the air down. Many heater boxes are like the
inside of a vaccum cleaner  dust bag. Cant tell from the out side,though.
got to split the case. Remember to get a new blower when you have gone this
far. $70 at our savior, BD. You can reduce the deciples a slight bit by
using a foam of spray foam on the exterior. Just takes the edge off the
sound. I think that resister pack produces a great amount of heat.That is
why they put it ino the box. The fan blows air over it and this heat is
added to interior heat. May be the same resister pack that is used on
tehexterior of thevan for  radiator speeds.They put it outside because of
the heat it generates. Usually all broken cermic on the pack located on the
outside. Heater box ceramic is usually good condition.