This happened to me once. There was a rock stuck in the little grate-like area just above the spinning impeller blades and they were smacking it as they flew across it.
I just messed with mine. I wiggled the intakes, pressed down, put the lid on sort of half-cocked, pulled the intake up, then back down.
Nothing. I can't make it make noise now. Figures, when you want something to go wrong, it never happens. I was thinking I could create noise and let you know what it was.
i got two like that. both my 350's did that, cleaned the impeller, tubes and even took it apart and hoses it down. no luck. put them in the closet and bought a fx5