The memory was replace however it did not solve the issue.
Today during some googling I found this document:ftp://untergrund.net/users/ggn/Atari%20 ... te0002.pdf
It describes audio distortion caused by the shifter.
The shifter in my mega STE is a revision D C301712-001. According to the document the IC U699 (under the PSU) should be disabled when rev D is present.
My mega ste is not modified (just checked). So U699 is not disabled and unmodified.
So it seems that I need to apply this mod.
Anybody in the forum have experience with this mod?