300tdi 130 1995, 480,000km
Symptoms:
1. Horrible whine noise at or above 5PSI boost pressure
2. Whine frequency too high for engine RPM, but too low for turbo
3. Wastegate would open with shop air (via step-down pressure regulator) at around 15 PSI
4. Wastegate actuator not leaking, as checked with soapy water.

(I have video of moving boost pressure gauge needle from inside vehicle and audio of under bonnet, but don't know how to post here)

Issue was:
1. The circlip ( STC3592 ) was broken from the wastegate lever shaft.
2. Two small tabs from the broken circlip remained stuck in the slot in the wastegate shaft, just protruding a little.
3. The wastegate actuator End Yoke ( STC4615 ) was getting occasionally stuck on the burrs created by the broken circlip.

Solution:
1. Removed burrs of broken circlip.
2. Lubricated shaft with high temperature neverseize
3. Reassembled.
4. Looking for suitable circlip and plan to make it a maintenance task every 450,000km !!