I have a Carrier Heat Pump system. I estimate it is 15+ years old. I had it serviced this past summer. (Reversing valve replaced & recharged). The unit has run well since. It now make a 'clicking' sound when the out-door unit is running. There is no mechanical interference that fan blades could be hitting. It also iced up severely. I ran the unit in the AC mode to defrost the coils and the clicking remained. What should I look for?

This one may need looking at by a pro, but you may have a bad low voltage connection going to the contactor. The reversing valve will also make a distinct click, and the defrost cycle will usually click on and off, so it may be any of those. A clicking sound is normal if any of those things are occurring at that time and are supposed to be occurring. The loose connection could be a loose wire nut, or even a bad contactor. Look at the low voltage wires first. I would also check the wires at the thermostat for tightness