If it's an intermittent click then it's possible the platter servo tracks are becoming damaged. The heads are on an arm that positions them on the rotating platters. It used to be that they were positioned in an absolute manner, as in they would go .0253" from the edge of the disc, in order to read or write a specific track. As density increased, though, they couldn't make the head positioner accurate enough for this to work, so no they have two read heads on each head, one is for data and one is called the servo track. The heads go the to approximate location where they need to be, then the servo track head is read and the head is adjusted until it's over the desired track.
The servo tracks are preencoded at the factory, and cannot be repaired. But they are just as "writable" as the data track, so if the drive fails in some minor way and allows the head to write over a servo track, then finding that track can be a problem later.
Further, the heads don't touch the platters. If they did they'd scrape off the molecular thin layer of magnetic material on the platter. The platters must be spinning fast enough to create an air current over them which the heads float on, just a tiny fraction of a millimeter above the platter. If the platters slow down too much, the drive senses an impending head crash (ie, head falling onto the platter(s)) and it slams the head to the side of the drive into the head parking position, which is off the platters. This is the type of clicking problem that a short time in the freezer will often overcome briefly.
Some drives also park the heads briefly if the servo tracks seem weird, in order to reset their positioning knowledge of the head.
As such, the clicks suggest your drive is damaged, and you cannot expect the drive to last much longer. Either your platter motor bearings are dry and the motor can no longer provide the consistent speed necessary to prevent head crashes, or the servo tracks are starting to become too damaged for the drive to rely on.
Back everything important up and replace the drive.