When I first bought our unit, the ps belt made no noises. Had the belts and other misc items performed and I got for my trouble a ps belt squeal at slow speeds. Had another new belt installed, and had the same squeal. Had the idler and tensioner pulleys replaced, the noise did not change. Then in my infinite wisdom I told them to change the pump. They said it wasn't the pump, but I can be stubborn, and they obtained the only one on the west coast from San Francisco at their cost of $891. It seems the pump on my unit came from Germany, and they were extremely proud of it. The noise remained, then I decided to compare belts. They are not made the same. The original OEM belt, the groves are taller and skinnier, then any of the replacement belts. They said the belt was worn out. OK, good argument. But, after 3 + years, the squeal is becoming less and less, making a better argument for my case of the fact that the belt was indeed made differently, and is now fitting the grooves better.
All the belts were matched up to the original belt for length and number of grooves only. I even went to Ford and got the belts that matched up by number to the OEM belt I had in my hand, of course the original number had been obsoleted by Ford. Their replacement belt was made just like the aftermarket belts I had bought combating this problem. One other thing, this belt also matched up to a CAT C9 belt which coincidentily has been discontinued.
I have as a former mechanic encountered this very same thing with certain overhead cam timing belts, aftermarket belts making noises compared to the OEM belt.
Whats it all mean? It means if you know the belt, pulley, tensioner and everything else are in good shape - live with it. As the miles increase I am fairly certain the belt will wear in and quiet down. Of course, in 10 years or 50K will probably have to deal with the squeal again and have everybody stare at you as you slowly turn into the RV park.

I have learned to ignore them, and as I said the squeal is becoming less and less. Just got home from 4500 miles and 26 days and only heard it once.
This is all just my experience, hope it helps.