This past week I began to hear what sounds like a whistling / chirping noise when I let off the gas after cruising at highway speed. It goes away when I get back into the throttle and I don't hear it when the truck is not moving. Not sure if it's engine or transmission related but I'm wondering if it could be related to the cold temperatures we've had this week. My truck has 35000 miles and it's a 2001 with the 5.4 liter. Any ideas?

 

Sounds like it may be a slipping belt.

 

I considered that it could be a belt, but I only hear the noise when the truck is moving. If I rev the engine when the truck is in park, the noise is gone.