Best guess is a linkage issue. When it's in a hover all the surfaces are more or less unloaded. You might be able to isolate it. Hover with full aile deflection one time. Next time add rudder for a split second to see if it stops , then try elev the same way. Some early production Funtanas's were lost due to aile flutter. The early run had a weakness in the aile mount. The later run of birds they boxed in the aile mount and cured the problem. You aren't getting flutter in a hover but a little bit of play on a surface that big can be a lot of movement.
David