I think the easiest way would be to simply turn the Drobo off and rotate it around. Then, with a flashlight, power the Drobo back up and watch to see that the fan is spinning. During the boot-up phase, it cycles the fan up. After the initial boot phase, the fan slows/turns off. You should hear a definite change in the clicking frequency/disappearance of the noise when the fans speeds/slows. That’s the first step I would take.
While my knowledge of storage/storage methods is nowhere near some people on this board, I have experienced enough drive failures to know that clicking can mean ‘failure imminent — do something dummie’. Then again, isn’t this the Drobo’s intended purpose — to have a RAID array survive with a single drive failure? Maybe let it do it’s thing and disqualify the drive. Do you have a spare handy to swap out when the time comes?
Good luck. And whatever does happen, please do post back what the eventual outcome. Sometimes a noise is just a noise.