It sounded to me like the beeping that a truck makes while it reverses, or the warning tone before a massive gate/door closes. We hear someone (
Datu? A zombie?) scamper off and do something, then *beep beep beep*, and a motor starts. The sound of the
generator re-starting? Although there's also a whirring sound that could be a generator winding down...
If the genny isn't working then
the colony might be in trouble, because Datu turned it off while they were trying to get the windmill running. And isn't that how they've been getting water?