Most of the older turbos whine as they spin up. It's just the turbine spinning at a high speed broadcasting some of that noise through the cases and out under the hood. Ever hear a Saab 900T take off? They really make some noise. I think it's cool- but then I've spent a lot of time around high-speed rotating machinery.
I used to think that the noise also might project from the airbox. However, hearing my friend's '89 240/B230FT conversion, which has a small, homemade "cold air" intake setup, I don't think it's that; the car makes kind of a WHOOSSHH sound when you get on the throttle and a BLAATTT when you decelerate, all intake noise.
It's true that the newer designs (the Mitsubishis) are much quieter. Couldn't tell you why other than they are a little smaller and have the benefit of another 10 years of optimized engineering behind them.
BTW it's not uncommon for turbos to spin at speeds of 20,000 rpm or more.
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Rob Bareiss, New London CT ::: 'Roterande Fläkt Och Drivremmar!'

Check also that the compressor bypass valve is on tight. There are two allen screws that secure that often come loose.

--Paul
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Paul '89 740 Turbo 102,000

I hear the turbo whine on my 740 as I did on the 780 I drove over the weekend. I can't hear it on my 240t or my mother's XC.


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don hodgdon '89 744ti, '81 242t, '71 D-35

As far as I know, the whistling sound is normal when the turbo spools up. Mine does it, and my turbo appears to be functioning just fine. The whistle that mine makes *seemed* to get louder when I switched from Amsoil 10W30 to 10W40. I can't explain that. Maybe it's just me :) I believe the older Garret turbos make a more pronounced whistle then the Mitsu turbos. You may want to double check for air leaks though, just to be sure. All appears to be ok, upon inspection of my turbo and related hoses......
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Chris. Halifax N.S. '91 745Ti, 274K km.