Hello,
thank you. I checked my fstab and there was swap and /home still mounted on my harddisk. I removed them and everything was suddenly silent .
Thanx for the great help!

Thanks for all the hints here. This works great.
But somehow my harddisk is still wakening up after about 5 seconds sleep mode. Does anybody know how to make this complete silent? Why is linux still trying to access the harddrive? Maybe I should mount /var also as a tmpfs filesystem?

Thank you