guys you made my day. Thank you so much.
I thought I go crazy with my Bluesound node 2i and the static noise.
short story. I bought new home cinema speaker (Canton Chorus SL 516.2 higly recommended) and divided the stereo and the home cinema system. Therefore I had to reorganize the devies and the cables. After that i had these sh… noice when play music via ROON and the Node. If I played music via blueOS and the node anything was ok. So I thought, this must be a problem with the Node or the cables.
And then I found this thread. 
Stop! Didn´t I change the admin user a few days ago on the QNAP Nas? I did but I didn´t changed the login in the bluesound app.
After I changed the login and made a new index everything is fine.
What a crazy issue. Normally a technican like me would say the Node or a cable is broken. Replace it.
So, thanks again. Great community 
Stefan