Hey Ed....on my 1997 960 105K I just updated the timing belt and serpentine belt. Funny, as now I too have a "whistle..."
Unfortunately there was a wrong alt. unit (see my 12-24 post) for whatever reason(s) which has been corrected and now I have high rpm whistle.
My mechanic tenatively isolated it to the pulley tensioner arm of the serp belt, which tensions with RPMs. Kept the old one on, as mechanic said it was fine, hydraulic unit, so no need to replace. He laughed and said that the whiny-whistle is just an exhaust echo, so I said "Oh yeah, then let' hit the road then ace!" "Oh, so it's not exhaust, huh?!?"

I can see the tension wheel is 'weathered' from use and not perfectly even, but no problem working, seems to be the source of the whistle....
He suggested it would pass over time/mileage and stop, but we'll see!
If it ain't quiet too soon I'll pop in a new tensioner and give listen.

Good luck Ed, and please let me know what unfolds as maybe it'll help me!!!

mikesullyvan@yahoo.com