Update: Something finally decided to give up - I heard a loud pop followed by a short frying sound just a few hours ago. Surprisingly, the machine continued running, so I shut it off, pulled out the fan and CPU, and inspected the mobo closely around the area where the sound was coming from. There seems to be no damage to any components, it all looked like it did before. I reinstalled everything and turned it back on, it still works (doing some priming now) but so far I haven't heard the sounds nor noticed anything else weird.