[sdiy] High pitched noise on codec problem

[julian schmidt]

Hello list,

I have a problem and need some help. ;)

I've build a STM32 board with a WM8732 codec.
at the moment I'm just using the left channel.

With the gain up on my mixer I noticed a high pitched noise on the output.
Now the strange part - as soon as I touch the unused right input 
channel, the noise slowly decays (maybe over 500ms) and is completely 
gone. The same happens if I just hold a piece of wire to the unused 
input pin.
Wiring the input to GND does not help, neither a resistor or cap to gnd.

the left input does not show this behaviour.
I can leave the left input unconnected as well and the noise completely 
vanishes as soon as i touch the right input pin. touching the unused 
left pin has no effect.

My input stage to the codec looks like this
http://i.imgur.com/Fa9KTe0.png
where R_IN is connected to the input pin of the codec.

any hints in the right direction are welcome.

cheers,
Julian