Originally Posted by Redneck2345
well the correct way sucks
Fighting with HVAC boxes in the truck sucks. lol
I always remove them completely if there's no access panel for the core.
That foam is there to keep the thing from rattling around. Though mostly just irritating to listen to, plastic certainly can and does rub holes in metal parts over time, especially thin aluminum like a heater core. Go to the auto parts store or home depot and get some 1/4" x 1" dense foam weatherstripping and wrap the core. Add layers if it still wiggles easily after the top of the box is reinstalled.