You don't recall this happening last year. What could it be?
Let's look at one of the potential causes. Cable tension.
Specifically the loss of tension in the rear gear cable. This is more likely the cause of clicking noises if the cable is fairly new.
Sometimes when a new cable is installed, there is the chance that the cable will stretch just a small amount after installation. When this happens, the movement of the derailleur is misaligned and causes the chain to sit in a position either above or below of where it should be between the cogs.
This is why I tell clients for whom I have installed a new gear cable to come back after a week or so of riding, to have the gears re-adjusted in order to compensate for a potentially stretched cable.
The easy solution while you are out on the road/trail is to use the adjuster on the derailleur (or on the shift lever if there is no adjuster on the derailleur). What we need to do is to increase tension to the cable, just a tiny bit. The adjuster rotates using finger pressure. A small bit of movement of the adjuster counter-clockwise is all that is necessary. Perhaps a measurement of 5 minutes (assuming you were turning a clock face) Adjust, test and re-adjust if necessary. Always in very small increments.
This simple adjustment is usually all that is needed to get your shifting back in order.
Failing that, be sure to take your bike to a reputable repair shop if the problem seems problematic or greater than might be caused by cable tension.