My hDC3 18" started making some godawful noise a week ago, sounded like a piece of tinfoil got in the box and was being tinselled up everytime the bass hit. UI pulled the usb expecting tinfoil to be in my box lol or my voice coil unwound. But everything loks perfect, coil looks like it did brand new... but if you tap down ont he cone.. it makes a large tinfoil like ratle.
Heres the vid, may be processing for a few minutes
EDIT: aparently you cant hear the noise when I had the camera above the subs, you can hear it @ 20 seconds and on though, sounds more like tinfoil ball in real life