I had something almost like that on my old gas unit. What I found out, was were the dash topper is mounted to the fiber glass nose, the screws used at the factory was too short, and there were only 4 with a sealer holding the dash to the fiber glass. I removed all of those screws, cleaned the old sealer, opened the top from the fiber glass, put in new flex sealant, and almost twice the length screws. I put in about 12 screws and put sealant all around each screws. I then went to auto zone or any auto parts store, and bought about 6 cans of under coating (rubber type), and sprayed all around (under the dash topper), any cable or wire that ran through the fire wall area.
No more yelling while driving, and no more outside air coming in. AC works better in summer and the heat feels better went it's cold.
Good luck, hope this helps.
2008 36'QSH Phaeton