It could have been the string rubbing the nocks, or the string stretching, but it could have been a splinter or the handle separating as you mentioned. It looks like your fade thickness increases before your glue line on the riser (as it should), is that the case? If your handle area is done correctly then i wouldn't think the handle is separating, but it's possible.
CLOSELY inspect the bow all over for any splinters beginning to lift or cracks, check the glue line on the riser. If it really was a "tick" sound, that usually indicates a wood separation somewhere. Restring the bow and check all over again for the slightest separation, which may simply look like a line in the grain/growth ring. Hold the bow in front of you sideways and with the back facing you and pull a little on the string and inspect again as the bow flexes, especially the fade area.
The only time i've had the tick, i eventually found splinters lifting, sorry to say, but one can hope!
