I don't see any IO errors in the logs, nor anything really corresponding to disk errors. Also disk utility reported no issues with the disk.
I did see these in the logs when system activity happens on idle:
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Jan 4 20:55:31 home-imac.elisa librariand[324]: unable to get mobile documents URL
Jan 4 20:55:31 home-imac.elisa librariand[324]: error in _handle_client_request: LibrarianErrorDomain/10/Unable to configure the collection.
Jan 4 20:55:31 home-imac.elisa librariand[324]: MMe quota status changed: under quota
Jan 4 20:55:31 home-imac.elisa com.apple.PassXPCService[1285]: PAPassValidator: _signatureIsValid failed to load signature file
Jan 4 20:56:01 --- last message repeated 6 times ---
Jan 4 20:56:11 home-imac.elisa CVMServer[102]: Check-in to the service com.apple.cvmsCompAgent_x86_64 failed. This is likely because you have either unloaded the job or the MachService has the ResetAtClose attribute specified in the launchd.plist. If present, this attribute should be removed.
Jan 4 20:56:11 home-imac.elisa CVMServer[102]: Check-in to the service com.apple.cvmsCompAgent_x86_64 failed. This is likely because you have either unloaded the job or the MachService has the ResetAtClose attribute specified in the launchd.plist. If present, this attribute should be removed.
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Jan 4 21:03:08 home-imac.elisa mdworker[1342]: Unable to talk to lsboxd
Jan 4 21:03:08 home-imac.elisa sandboxd[1343] ([1342]): mdworker(1342) deny mach-lookup com.apple.ls.boxd
Jan 4 21:03:08 home-imac.elisa mdworker[1344]: Unable to talk to lsboxd
Jan 4 21:03:08 home-imac.elisa sandboxd[1343] ([1344]): mdworker(1344) deny mach-lookup com.apple.ls.boxd
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But the really annoying thing is that things like scrolling this discussion page up even a few pixels or moving the cursor while writing to this text box triggers disk writes (page ins). So I'm starting to wonder this is just a combination of an unusually noisy disk & some odd disk paging behaviour.