yeah. its normal with the new gear box. when i installed my new gearbox, i got the same problem. HEAT!! i taught its the heater core OR some engine air leakage entering the cabin, but NO, it was the gearbox issue. The whole area gets heated quite good enuff to feel after 1 hour of continuous drive, its when the gearbox comes in REAL-Warmup action!! Physics:Heat is energy. There is A-LOT energy there inside the gearbox, so it produces HEAT, especially new gearbox. the only 1 solution to avoid this issue is to double carpet OR aluminium foiled thick foam/fiberous coated sheet to the heating area of the whole gearbox in the passenger compartment, not the underbody. There is no such covering aftermarket or OEM solution for the underbody, which many mechanics will tell you to get the gearbox shield, that is useless term. This will need removal of front seats & the whole dashboard material, including other misc. plastics attached to the front area.