The last few days its been on the cooler side up here in the morning. I remote start the truck for a few minutes and get in. Put the key in and turn and then turn the heat on. Just as the fan start to come on, I swear I hear a purring noise coming from the dash that sounds just like a cat !! It does it twice and then not the rest of the day. Its sort of funny really but what the hell is it?
Marty

 

It more than likely is the blend door repositioning itself based upon the your EATC setting. *Would be more pronounce at a cold/cool start-up.
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