A weird one, and I haven't read anything like this in any forum. Sounds a bit like a blown sub or something loose or rattling, but less noticeable on high volumes. Otherwise produces bass as it should.
Tried to move membrane with my hand. When there is no power, it moves smoothly, but when amp is on and without anything playing, sounds and feels like coil is rubbing against magnet in one certain spot. It doesn't make sense. If coil was bad, I think it should rub all the way.
There is no damage, nothing loose on plastics, cone and membrane. I even bought epoxy, hoping I could glue it back together, as this was a common problem and successful fix for many.
I'm almost sure there is nothing I can do, but maybe someone have any idea or possible fix?