Audi A3 2.5 TDI Engine Noise - 2003

Also mein Problemkind ist eine Audi A4 8E B6 2,5 TDI mit 217000 km. Engine identification code: BAU.
The annoying habit he has that drives me crazy is as follows:

When driving at low RPMs, a noticeable grinding/whistling noise can be heard in the 1500 to 2500 RPM range, specifically when the turbocharger kicks in and builds maximum boost pressure. The noise is most noticeable when driving uphill, but it can also be heard when driving downhill using engine braking within the aforementioned RPM range. The turbocharger was replaced by the previous owner approximately 30,000 km ago. I have visited three VW/Audi workshops, and they all stated that the noise is normal. However, the noise was definitely not present when I purchased the car. As far as I understand, blowing air back into the air filter when downshifting is normal due to the presence of a VTG turbocharger.

Here's what I've already done:
Gewechselt habe ich das Ladedruckregelventil, einen Ladeluftkühler, Abgasrückführventil, Das Ventil welches die Druckdose des Abgasrückführventil steuert. Habe mir einen Stopfen mit einem Ventil gebastelt und die Ladeluftstrecke abgedrückt und seit dem ich alles gewechselt habe ist diese auch dicht. Die Unterdruckleitungen zum Turbo und überall anders hin habe ich ebenfalls kontrolliert – sieht gut aus -. Schlauch zwischen Luftmengenmesser und Turbo habe ich auch schon gewechselt da ein Riss unterhalb der Schelle zu finden war. Ladedrucksensor habe ich auch ausgewechselt. AGR is also new.
Error memory is empty.
Power loss... well... at first, it seemed like the car had better low-end performance.

I once logged the boost pressure and mass airflow sensor readings, but I can't make sense of the data.

Please help, I'm open to ideas, tips, and anything else you can offer. I'm grateful for any assistance.
Best regards,

Take a close look at the flexible pipes of the EGR system and the catalytic converter.

Hello, thank you for your reply.
I've already replaced the AGR valve (it was leaking). I also checked the intercooler piping for leaks, but I didn't find anything. I could replace the crankcase ventilation (PCV) valve just in case. However, my research indicates that this engine uses a cyclone separator in the PCV system, which is not really prone to failure. When I remove the dipstick or open the oil filler cap with the engine running, I don't hear any hissing or feel any vacuum. Could the PCV valve still be faulty and causing these noises?
If anything in my assumptions is incorrect, please let me know. I'm just a hobbyist mechanic .
Sure, here's the translation:

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I once created a diagram from the data log of the air mass sensor.

I have also created a diagram based on the boost pressure log.

First, please enter your VCDS license information in your profile and send me a private message.