Thanks for all the replys so far!
and sorry for coming back late, but I had to do some more investigations...
1. the noise seems to occure on each real track (even Organs...)
2. The noise also occures if files are rendered on a different computer

Strange...
Off course I could gate it away, especial in the beginning of a song, but at the end of a song, when a chord slowly fades, the noise is more and more prominent.