Sounds more like the cooling fan than the turbo, turbos dont really roar but the cooling fan does make a roar especially now in summer, and then the roar disappears when the coolant temp goes down and the viscous clutch disengages.

__________________
87 300d Turbo 241K with original #14 head (blue)
87 300d Turbo 198K will run great again someday (silver)
84 300sd sold ( buyer drove it to Paraguay )