I have a 1992 Oldsmobile Cutlass Supreme S. It has the 3.1 MPFI, 150k on it. It has lately been making a loud whirring noise under the engine over by the drive belt, and when I hit the brakes to a complete stop, it idles very rough, and randomly tends to stall (die). It sometimes backfires really loud after driving a while. I've heard three different things, it could be the steering belt, timing, or alternator. HELP!
This isn't something I know much about, but if I were in your shoes, this is what I would think about first.
Sound could be coming from bad bearings in any of the components along the drive belt. Add in the rough idle and dying when a sudden shift within the engine is made (braking, causing the engine to slow down and sensors compensate) makes me think Timing, Cam or Crank bearings. Those would be things I'd look into first. And rule out what I could.