I just had a customer who is having a similar problem. He has a GFI in his garage that is tripping randomly, say 1 or 2 weeks. It'll just trip with no apparent reason. I checked it out 2 different times. One time I found an outside outlet with loose wires. Thought that was the problem, since he had a fountain pump plugged into it, and thought maybe the arching was causing it. Turned out it still tripped actually the same day that time several hours after I left. I went back, and found a drop cord that was going under his house had some bad places in it. We replaced the drop cord. I then went up in his attic to check some "home wiring" he had done, placed a few wirenuts on joints he had made without putting any on them. Left that time, told him to disconnect the fountain pump if it did it again, and see if that made any difference. Left and he called me back today saying it tripped in about a week after that with no load on the circuit. Since it is getting costly for me to go back each time, I told him to change the GFI outlet, (which he told me was a new one to start with). I suspect the wiring he done may have something to do with it, but without tearing everything apart, I don't know. Hoping it's just the GFI outlet. The reason I didn't change it to start with, is he told me it was new. But his idea of new, I think is a year or two.??? Very odd though that it trips randomly with no load connected to it. Anyway I would try the GFI first on your problem... I also had one, one time that would do a lot of clicking, brand new from HD store. I had to replace it. Hated to though. I had to travel probably round trip of 30 to 40 miles to do it:( Hope it helps.. Steve
This sounds to me like a minor neutral-to-ground fault on the load side of the GFI receptacle. Not necessarily enough to trip it, but enough to make it "think about it". I've found things like cobwebs in outdoor receptacle boxes to be just enough to cause phantom problems depending upon the humidity level. This minor amount of leakage is impossible to detect in testing.
Then again, how much cable length is on the load side of the GFI? If it's more than 250 feet, that alone is enough to cause them to malfunction even with no load. 250 feet sounds like a lot until it zig-zags all over the house to hit all of the bathrooms & outdoor receptacles. I don't even like to see 200 feet on the load side myself.
Fountain pumps are notorious for causing nuisance GFI activity. Inherently, electric motors have a small amount of leakage to ground and they will have you pulling your hair out since this leakage only occurs while the motor is running. Any kind of small motor load can cause this wacky activity on a GFI though, not necessarily just pumps.
I had a problem where the GFCI appeared to randomly trip in the early morning. Cause was located to a scratched wire in an outside outlet downstream. It was related to the dew in the morning.
I had another one that would randomly buzz. I thought the noise was from the nearby trash compactor and unplugged it. The GFCI then faulted later by arcing. I replaced it and when I looked inside it, you could see a damaged coil.
I had a similiar problem last week at the park our trailer is in, floating pump in the middle of a small lake. Gfi would trip immediately but when connected to standard breaker (as a test) wouldn't fail, turns out when we pulled the pump and cord (no mech protection) there where nicks in the cable and part of the nuetral was exposed in one place and part of the ground in another.