Deskstar NAS 10 TB periodic loud clicking/thumping


I just bought three Deskstar NAS 10 TB drives and am concerned about the very loud clicking every few seconds, especially when the drives are idle. The rotation sound is very quiet and I think that the regular seek noise is also quiet. I have zeroed the drives with DD and run the smartctl long test without any errors reported. I haven’t taken them into service as I fear that they are about to break, all three of them.

Is this normal behaviour?

(I have 12 Deskstar NAS 4 TB 64Mbit and 4 Deskstar non-NAS 4 TB drives that are all working flawlessly and quietly since a couple of years.)

Best regards