ReadyNAS Pro Business, large fan makes fast ticking noise

Usually when I get that noise in a PC it is because a wire is out of place and is being struck by a fan blade. You might want to open the case and check to make sure some loop of wire that you cannot see from the outside is not intruding into the fan space. As long as you do not remove any screw that is covered by a "Do Not Remove" sticker, you warranty will be unaffected. I am not familiar with the construction of the 6 bay frame, but on the 4 bay frame, you remove four screws holding the two side plates in place, slide them slightly to the rear and remove. Then two screws holding the top are removed and the top panel is removed. According to the images on Newegg for the Ultra (same chassis) you can then look straight down behind the fan and see if one of the PSU wires is intruding slightly into the fan space. You do not need to physically remove the back plate holding the large fan in place.