Avantime Turbo Boost Issue

Sounds like a boost leak. That whistling/hissing is a good clue. The actuator controls the turbo wastegate, which manages boost pressure. If it's faulty, it could cause those symptoms. Have you checked for any obvious leaks in the turbo hoses or intercooler? A defective electrical bypass valve could also be the culprit. That part is also called diverter valve. It's located on the turbocharger and recirculates excess pressure when the throttle is closed, preventing compressor surge. Do you have access to an OBD2 scanner to check the fault code?

I have visually inspected the hoses, but I did not find any obvious cracks or leaks. I will inspect them more closely. I do not have an OBD2 scanner. How can I check if the electrical bypass valve is defective? Is there a specific test?

Without a scanner, pinpointing the exact issue is tricky. A faulty bypass valve can cause similar symptoms to a faulty turbo actuator. You can check the bypass valve diaphragm for tears, but you'll need to remove it. Given the symptoms, it's probably best to get it scanned to narrow down the problem. Your local workshop should be able to diagnose the issue more accurately. It might even save you money in the long run by avoiding unnecessary parts replacements.

I took the car to a local workshop, and it was indeed the boost pressure actuator. The bypass valve appeared to be functioning correctly. The total repair cost, including parts and labor, came to 250€. The car is now running smoothly again.