Hi, I realize this is an old thread; however, my searches landed me here. I just wanted to share that I've been experiencing the same thing on two of my FMMs, which started suddenly.
I believe I've narrowed it down to an AB cable (a long one running through the dang ceiling!). However, in my t/s endeavors with Neptune, I learned that there would be an update (soonish) that may also cause the same issue (beeping FMM).
Neptune said they would be adding the ability for users to manually invoke the alarm on an FMM. They were concerned that my FMMs somehow received an update and that somehow I had turned the alarm on. However, that isn't the case, so I just wanted to share these tidbits. On the one hand, it sucks for FMMs to randomly misbehave (especially if a cable goes untouched for months and months) On the other, we will eventually get the ability to activate FMM alarms manually via programming soonish.