We had a lot of rain recently, and I noticed that when I turn it over after it's been rained on all night, I hear a sound that sounds like a belt slipping, for maybe half a second. Then it's gone, no more problems until the next time it's left all night in the rain.

Could my serpentine belt be slipping because it's wet?

 

could be due to moisture and condensation yes. check the belt tension to see if its within spec, if not, adjust it accordingly.

 

I haven't a clue what the tension should be, how to test it or how to adjust it. I wish Haynes made a manual.

I called my dealership, they suggested that I bring it in for a look-see.

 

I think it's just a spring loaded tensioner. I don't think it's adjustable.

 

^^^^ that.

 

I had a Ford do this and it was because the belt was no longer soft. The rubber had become kind of hard and slick. Dayco and Goodyear make good replacements, but a GE should not be that old. How many mi/km on the car?