Read timeouts, Obvious drive ticking -- no logs, no SMART

I have an Ultra 6 with 4 2Tb Barracuda LP's installed.

Lately I've noticed some pauses in streaming reads from the NAS (my iTunes library is hosted on it) associated with an obvious head ticking from one of the drives.

The problem is, I can't ID which drive is bad from the logs or GUI. All of the SMART data checks out, I ran the extended SMART tests from the boot menu -- nothing. There are no errors in the logs to indicate failure.

The Pause/Ticking is happening with increasing regularity, and I'd really rather avoid an emergency situation. Does anyone have any tips for locating which drive is going, without methodically swapping each one out?

Status page:

	Device 	Description 	 	Status 
	Disk 1	Seagate ST32000542AS 1863 GB , 38 C / 100 F , Write-cache ON		OK
	Disk 2	Seagate ST32000542AS 1863 GB , 38 C / 100 F , Write-cache ON		OK
	Disk 3	Seagate ST32000542AS 1863 GB , 36 C / 96 F , Write-cache ON		OK
	Disk 4	Seagate ST32000542AS 1863 GB , 35 C / 95 F , Write-cache ON		OK
	Fan SYS2	897 RPM		OK
	Temp CPU	24 C / 75 F [Normal 0-65 C / 32-149 F]		OK
	Temp SYS	34 C / 93 F [Normal 0-60 C / 32-140 F]		OK
	UPS 1	APC Back-UPS ES 700, Battery charge: 100%, 55 minutes		OK

Details

Model:	ReadyNAS Ultra 6 [X-RAID2]
Serial:	<redacted>
Firmware:	RAIDiator 4.2.19 
Memory:	1024 MB [DDR2]

IPv4 address: 1:	192.168.1.197
IPv4 address: 2:	Not Connected

Volume C:	Online, X-RAID2, 4 disks, 66% of 5548 GB used