What does the SMART info show for the drives - are there any reallocated
blocks? A large number of those scattered over the disk would certainly
cause seeking for both reads and writes.

I will check the SMART this evening but I don't think that this is causing
the seek. The sound is very constant and for the whole time of syncing  
the array.
I will also run a dd on the disk to compare.

It's also worth checking whether there's anything else that would be
accessing the disks in the background (monitoring/indexing/etc).

Unlikely because I have not yet created a filesystem after setting up  
the RAID4.

I can't think of anything else that would be causing reads to seek - SMR
disks or write-intent bitmaps would only affect writes.

Exactly

Is there any way or tool to monitor all disk read/write commands - not only
the count or amount but every access with LBA and length?

Patrick