YOu have an 85... it IS the ARV. I know everyone will tell you belts, but I bet it's the ARV since it sounds like you noticed it was boost related. ARV= Air Recirculation Valve... its an emmisions only devise to allow a large enough flow through the EGR during certain boost levels... it HOLDS BACK boost and limits power when it's open... I took it apart when I had the turbo off and plugged it permantly, but the plug will do it.

See that blue line going into the side of the turbocharger compressor?? Pull it off and put a BB into the rubber connector and smile as the squeeling goes away.

__________________
1985 300D Mango Green with 16" Pentas, Vogtland springs, Euros/ clear corners, 234k, lots to come soon....