I've had this in mine. I use sand as a substrate in that tank and a bit may have been caught behind the impeller.
Really easy to fix. Drain the filter and undo the housing with a long phillips screwdriver, pull off the housing and you'll see the impeller. If I remember rightly, it just pulls off. I used a cotton bud to clean everything out and reassembled. Problem solved.
The impeller is the only moving part in a FX5 so nothing much can go wrong. Hopefully!