The squeal sounds like a bearing, possibly an idler wheel, or alt/water pump/PS pump.
Pull fan, remove belts, and examine all the rotating machinery on the front of the engine. Be sure to mark the operating direction of the belts, so you can put them back on correctly.
The idler pulley bearings can be cleaned and repaced, if not worn or scored.
Ed in San Jose '97 540i 6 speed aspensilber over aubergine leather. Build date 3/97. Golden Gate Chapter BMW CCA Nr 62319.