This happened to me recently, almost right at 100k miles.
Pushed a BUNCH of fluid out of the reservoir cap and had some nasty noises coming from the pump plus noticed the belt was nearly bald in places. Also the fluid was almost black.
Got some Seafoam brand PS flush, douched the system 2x with it, drained as much out, and filled with "quality" PS fluid. Works like new again and no noises from the pump. Figure the flow valves in the rack were clogged up and causing the pump to work overtime.