The 85 model has a pop-off valve on the intake sort of like the waste gate on the exhaust. When the throttle is to the floor and released the slowdown of the motor causes the air to back up on the turbo and slow it down. The pop-off allows the back pressure to by-pass.
The device is at the front of the intake, near the top of the motor, easily accesable. Try pulling the vacuum line off and pluging it. The whistle will go away. I presume it has to do with the transducer that controls it. Some are worse than others.
Bosch Master, ASE Master, L1
33 years MB technician