Hello,
I have similar problem with my Astra 1.6T, made in 2018. About three months ago it started to make strange high-pitched noise when idle when the engine is warmed up (sample audio recording attached). When I touch the accelerator and raise RPM just a tiny bit, the noise stops for a moment. The same noise is also present when shutting the engine off, the frequency of the noise drops for the moment when the engine is stopping (for about 0,5s-1s). I will try to record it, but it sounds to me like the recording from Neo2001. Next month I will try to sort it with the Opel dealer, however I'm a bit skeptical about it. My assumption is that it could be related to some pulley either on the timing belt or accessory belt, maybe bad tensioner.
Hallo @westler,
der "Vogel" bei dir im laufenden Motor hört sich ja auch nicht gut an.
Melde dich bitte wieder hier, falls dein Opel-Händler etwas zu dem Pfeifen beim Abstellen des Motors gefunden hat.
LG Sven