Like Socketman said. I would undo the screws on the tweeter and check the connections. it is simple to do. if they are secure on both the speaker driver end and the crossover then you can investigate a bit further. Has the speaker fallen over or taken any sort of damage? I had a cat attack a speaker once and nocked it over. It gave a simular sound. turned out that the crossover board got wacked by the magnet from the back of the woofer and cracked. it was one of the printed circuit board leads that was damaged and just needed to get heated up and a bit more solder added.

You didn't say if it was both the speakers doing the same thing, or just one speaker.

if it's just the one, then you can try swapping the tweeter on one speaker with the other. if that fixes the problem follows the speaker then you'd have your answer.

Anthem: AVM60
Axiom: ADA1000, LFR1100, VP180, QS8, EP500, M3, M3comp
AudioSource: Amp One/A