Water on the belts may tell you something, but I am not sure what. Does this happen with the ac on or off, or both? Have you visually inspected the belt, and pulleys? Had squeals like this over the years, some were belts, some were pulleys, some were bearings,(including the bearing bracket on my 87 300e) some were oil, coolant or other fluid leaks that get burnt away after a minute, so they squeal disappears. I suggest you look and listen closer to the engine to narrower down the area of the noise.