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Glad you described it -- a low growl or grinding sound. And I recognize it... I had that once in one of my Volvos.
It turned out to be a bearing in the pulley of the air conditioner's compressor's clutch. Made the sound when the A/C wasn't on (when it's freewheeling) -- noise stopped when the A/C cycled on, but then I could actually see it wobble.
Now, with you, it may not necessarily be the A/C clutch -- it could conceivably be anything with a bearing, such as the P/S, or even the water pump. A clue though -- I could actually see little slivers (like the "dust" or filings around a grinder when you grind a piece of metal) under the A/C clutch -- so look around on the engine and on the splash pan below to see if you see such an accumulation, to get an idea what has the bad bearing.
Another suggestion, at least to test your A/C or P/S as possibilities, (1) turn on the A/C to see if the sound changes, and have someone turn your steering wheel to, likewise, see if that causes a momentary change in the sound (when your P/S pump works harder). As for the water pump, you can't turn that on and off, unfortunately.
Alternatively, think of a stethoscope -- get a long hose, or long metal or plastic tube, and touch one end to the P/S pump, A/C compressor, and water pump, while holding the other to your ear. The sound intensity may also reveal where the noise is coming from.
And finally, if it's the A/C clutch, be leary if the mechanic says you need a whole new compressor. I don't know if it's different with yours (because you have a 1991 model, and it may have a different compressor), but when I had this problem (bad pulley bearing on the compressor's clutch) on my '84, my mechanic simply removed the bad clutch+pulley from the compressor and replaced it with a new clutch -- he didn't take out the compressor and I didn't have to pay for a replacement compressor, and didn't even need to recharge the system. So let him convince you that he can't replace just the clutch+pulley.
Good luck.
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