Clicking sound in front a/c - heat pump

I was out camping this past week and the temperatures were pretty cool. I had been running the heat pumps, front and back. The last night we were there, the front unit began clicking. It sounds like one of the fans hitting something as it's rotating. I would guess it's only clicking at a rate of a couple of clicks a second so it's not as fast as I would suspect a fan to be rotating. It's plenty loud and it is definitely a mechanical sound like metal to metal.

I haven't climbed up there yet and thought someone might have some input on what to look for. It may be obvious when I take the cover off but thought I'd ask anyway.