When my engine is running and it is cold outside (usually around 0C) I hear a high pitch whistle coming from my engine. I can't find any leaks in the hoses. The sound usually goes away once the engine has warmed up. Do I need to re-check and check again all my hoses, or is there something else that it could be?
<--- 'Runner rookie

 

mine did it too, then the water pump bearing seized. That wouldn't have been too bad except that the pump is powered off of the back side of the timing belt, and when the water pump seized, the belt got hot and basically decentagrated. It also melted a few pulleys in the process. I lost compression when the belt went. No damage will occur when this happens though. I was due for a new timing belt and water pump anyhow, so the only extra cost was the $40 in pulleys. If it was a honda, the whole head would have been done because of the valve to piston clash that occurs on those engines when the timing belt goes.
So anyhow, it might be the water pump bearing squaling, but it also might just be the fan clutch squealing on ya. I would probably have it checked out, regardless.
I have a 98 4runner.
Wes

 

I don't believe it is the water pump as I replaced it 20K ago when I did my timing belt.

Fan clutch you say . . . Would the clutch be engaged when you start up the truck? I know nothing about how the fan clutch works.