Thule AV100S with whistle tone

I am trying to find out what is wrong with a Thule amp. It gives a constant whistle tone of about 4kHz. Independent of volume or input selection. In both channels equally.
Anyone with an idea of the problem? Maybe it is interference of two high frequency signals from the clock?
Or display?
Another question: can someone find the technical reference or schematics of this amplifier?
Many thanks in advance.
 

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