What exactly does this tick sound like? Does it tick in regular intervals and then suddenly stop or does it decrease in intensity and tick rate over time until it disappears?
My #1 suspicion is that It might be just a fan with a bad bearing or something stuck in it. Often times your PSU fan speeds up under load and it only becomes audible then and afterwards while it works to cool the PSU under load.
WARNING: DON'T SCREW AROUND WITH A PSU IF YOU ARE NOT SURE ABOUT WHAT YOU ARE DOING, THE CAPACITORS INSIDE CAN GIVE YOU A DANGEROUS SHOCK. I AM NOT RESPONSIBLE IF YOU TAZE YOURSELF. DO NOT JUST POKE AROUND IN THERE WITH A SCREWDRIVER.
So, warning out of the way, you could unplug it and let it discharge for a good long time (2hours is good) and if you're comfortable around electronics open it up to check on the fans. Usually if it's a good PSU you can unplug them and test them individually. They usually have 2 or 3 pin connectors and run on either 5V or 12 Volt. (See the Label on them.)
First of all give it all a good air cleaning to get any dirt out. If you can unplug the Fans, try to spin them around a bit with your fingers or blow at them in the case if you cant get at them. They should spin very easily and remain spinning on their own for a good time. Check if you can reproduce the ticking sound somehow. (Use your PC case fans as a comparison.)
If one of the fans is ticking, you have a bad fan or it's just really gunked up with dirt/dust. At this point it's really up to you if you clean/replace the fan or get a proper new PSU.
If you're not sure about all this, after 2 Years It's as good a time as any to get a new PSU ( It costs you less time(money) anyhow probably )
Alternatively there could be a bad component or some parts cooling off after thermal stress that make a ticking noise when doing so. If that is the case, RUN RUN THE HELL AWAY FROM THAT PSU. (Or just unplug all the wires to defuse the bomb). 
/s
Either way If you're no longer comfortable using it in this state I would suggest you replace, PSU's are not something to compromise or mess with if you're not certain whats going on. If they fail they will generally either pop a fuse and die quietly or violently nuke everything in your PC.