I have a clicking noise emanating from the heater area behind the center of the dash and am wondering if anyone has encountered this and solved it. I'm guessing a vacuum leak to the blend door on the heater, No heat loss, A/C works fine, noise only occurs for less than a minute when truck is first started. The noise will go away immediately if I crank the temp control up to heat setting. Manual heater. Winter is coming, and I'm guessing if I don't address this issue now, I'll be working on it in -40c temps! TIA!