The trains are powered like subway or elevated rails, they spark as does the FITH trains on occasion. The squeal you are hearing is either two things- 1. New wheels rub the rails and they squeal 2. When the water is not running full power the tracks get really dry and the wheels squeak, also this can sound like someone is chewing gum really loud
When the ride is closed for fifteen minutes towards the end of the day (last few hours) that is typical, they are pulling trains (taking trains off). When you have 15,000 people on park you want to run four trains, but as people leave there is no need to run four trains, so you pull some (take one or two off). This helps the employees who have been spinning in circles for endless hours working multiple trains and this also helps save the trains (running trains around and around is hard on the machines). It’s not always a “problem” like so many people assume.
Also, a lot of the track has been replaced in the last few years and the trains have been re-built too.