Marshall MBC410 tweeter broken?

Hello guys,

Two days ago the HF driver on my MBC410 cab broke down during rehearsal. At least I think it's the HF driver. I'm using the matching MB450H head by the way.

When I play the rig on a normal level (master only at 2/10 ! ) the horn rattles, I know it is "horn-related" because the problem doesn't occur when I turn the horn of using the attenuator switch on the back of the cab. I can easily buy a new horn from the web, but i'm not sure it is the horn itself or maybe a problem coming from a crossover circuit or what not. I tried al the channels individually and blended (the head lets you do that) and it happened with all the different settings...

Does anyone know a way to pinpoint the exact source of the problem?

this is a picture of the horn that I took from a webstore:
http://a2.images6.thomann.de/pics/prod/234316.jpg

 

k, I feel stupid now...

somehow a small spacer ring (really tiny) got trough the grille and bounced into the horn. It got pulled by the magnet and was jammed between the cone and the horn unbelievable !

It could only be seen once the horn was taken out of the cab.
Almost spent 35 euros on a replacement.