Mine is a TS-209 and I had the same problem during the summer this year. The NAS wouldn't boot after I have had shutdown for about 3 weeks when being avay on holiday. I heard the same clicking sound you are talking about. I managed to make it boot without disks and then hotplugged the disks followed by a reboot. This restored everything back to normal
Now, 2 to 3 days ago, it stopped responding and I had to turn it off... Now, I can't boot it again - still the same low noise clicking sound, even without disks.
I'm gonna try out a new PSU like described here - I think that is the right solution.