The moment you know that the speaker foams need changing.

[rivetmaster]

I could hear them starting to go. There was a slight rattle, so I thought I'd capture the moment on camera. Had to pump up the bass a wee bit

 

[Eddy]

Got a FH 10 for free 5 years ago . Foams were already gone . Previous owner did that for me

 

[Hajidub]

Yeah, foams going or incorrectly newly installed will metallic rattle that voice coil. Weird though how increasing the bass, the movement of the speaker, helped. Might get away, for a bit longer, by removing and rotating the speaker 180 degrees, either way refoam is suggested.