My 86 900T has been making a "SHHHHHH" sound for about 3 years now when the turbo is boosting. I did not initially notice a lack of power, but now, going up steep hills at 6,000 feet elevation, I do notice a lack of power.
As part of a clutch hydraulic hose replacement (see earlier post/thread, if interested) I did take the turbo plumbing apart and took all the relevant boots to Mile Hi Body Shop in Denver, where they found two of four boots suspect and I bought used replacements.
No change.
Is it the Turbo or the Waste Gate? The plumbing seems to be just fine, as far as I can tell.
TIA,
Charles