I had a high pitched sound here recently too, and I couldn't figure it out until the other day. Turns out my washer fluid was too low.
Lately I noticed there is a faint high pitched sound coming from my factory head unit. It usually happens when the car is first started and the HU is OFF. If I turn the HU on then off again the sound goes away. It is a factory unit and the only aftermarket stuff I have is a JL 12W3 with a TMA 600w amplifier in the trunk. This only recently started happening. I've had the same equipment in the car for 5 years.
Need VAG-COM work in CNY? PM me!