It doesn't happen when the machine is moved, as is said by others, and it happens so sporadically that it is about impossible to reproduce. Sometimes the machine goes days without making the sound. Seems to happen most often, though, when the machine is hot or the fans are running. It sounds like a singular, non-repetitive "tick" or clicking sound. No performance issues whatsoever, and everything seems to be working in fire order. Fans work and turn on, machine doesn't get too hot, and other than that very rare sound, the machine is dead silent during normal tasks. Any ideas of what it may be?