Pressure release

[Groundpounder1]

Hello all. I have an issue that has come up and I couldn't find anything related on the forum so I figured I'd ask.

The last couple of days, when my truck has sat for several hours, I've heard a high pithced whistle sound for about a second after I start the truck. It sounds like a pressure release or something similar to that. I'm pretty sure it's not any belts squeeling.

My first though would be that my coolant lid may have a crack in it. I haven't had a chance to have the hood up while someone else starts the truck. I have never had any puking, that I can tell.

If that is the problem, could it be caused by some underlying problem? I don't really hammer on my truck. It is cold here but I usually let my truck heat up for 5-10 min before I drive it anywhere and even then, I go easy on her until it gets up to operating temps.

Any suggestions would be greatly appreciated.