I always hold a bar of soap to both sides of the belt while the engine is running.
I did that and it caused my belt to bawl and slip like mad. I ended up having to remove it and clean it to get the soap out. It may have been an error on my part, but it was terrible lol.
I had a similar problem to the OP awhile back. Same thing happened, washed under the hood and started heading a squeak. It was my idler pulley squeaking. I removed it, took out the bearing seals and repacked it with grease, and haven't heard a peep from it since.
You can make sure it is the idler pulley by spraying some silicon or WD40 onto the inside of it while the truck is running. If it goes away then you know it is that. Just imo.