Does the guitar go out of tune at all when you hear this 'click'?
It's very likely just a neck creak, no really worry as long as you don't notice any shifting in the neck.
It's usually simply caused in the neck pocket, you have the wood joined there under a lot of pressure and it can 'slip' ever so slightly when you change the tension (using the 'wang-bar') and it can make a sound like a 'click' or 'creak'.
My 520 did that when I first got it, I tightened the bolts some but that didn't fix it.
Over time it just went away, the wood settled, much like a house settles over the years. It will make noise for a while then just stop.
If it really bugs you, there's a simply trick to stop it, if it is indeed just a neck pocket creak; baby power, or better, plain talc.
Carefully remove the neck, (go to Rich's site for detailed instructions), and put some talc in the neck pocket, put everything back together and see if that solves the problem. DON'T over do it, just a dusting, something for the wood to grip, instead of slip . . .
Personally, I wouldn't bother with it, just let it click and see if it settles after a while, if not, try the talc, if the talc doesn't fix it THEN I'd suggest having the tech look at it again. (This time, show him what you're talking about right there in the store). :-)