At 111,000 miles I would detension the belts and check all the pulleys for any play, you are checking for loose bearings (side to side movement) and rotate the pulleys and check for any grinding feeling in the bearings.
At 100,000 miles it is common for these pulleys to go bad. When they fail they will disintegrate very soon after you start to hear noise and in most cases they will take a belt out with them and strand you on the side of the road.
You'll be able to tell if any of them have already been replaced by looking at them. If they appear to be original then plan on replacing all of them with the belts.
There is a very good Write Up on here for that job. It can be a pain to do but well worth it.
I had one pulley start to make noise and changed only that one because it was easy to get to, then within a couple of weeks I had two others making noise. To do all of them at once with the belts will cost you a little less than $200.00. That write up has a picture of all the pulleys and thier locations and the Dayco P/N's for the pulleys - can be cross referenced for Gates or NAPA.
Just click on the "Write Ups" tab on top of this page.
Also change the Supercharger oil. GM says it never has to be changed (bullshit).
So if the previous owner was having it serviced by the dealer then it more than likely never has been changed and is over due. Look in the Write Up section for the procedure and use the GM or equivalent SC oil (8 oz.).
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98 Riv, 102K Miles, EBC, LS1 Ft.Calipers, STB, Addco Rr Sway Bar, 180* stat., SSAC Headers, HF Cat, 1.9's,
105# springs, MPS 3.4, DHP tuned by the Deekster.