I know this does not help. It can be any number of things. It can be as simple as a piece of sheetmetal that has bent out of shape to the the bad news of compressor rod cap starting to knock. Do monitor your system to see if this clicking turns into a knock.
I do think an experienced tech would be able to pinpoint the cause for you. Obviously by posting, you may not know who they may be and since I do not know your general vicinity, I do not know if I can help you locate such a contractor. I do suggest asking neighbors and relatives, and double checking on Angie's list and Checkbook.com to see if the contractor is consistenet. I know of horror stories of relying on BBB or Chamber of Commerce, so I am sending you to consumer inspired sites. Of course, if you are in WNC, feel free to email me. :-)
There is a reality, that I feel it would be an injustice not to point out: 15y is the average servicable life for this piece of equipment. You have gotten a great life out of it. I suggest being proactive and start saving for the future for when it is time to replace this piece of equipment.