Wim,
When you say "swapped woofers, same result" you mean you moved the woofer from the left speaker into the right speaker, and vice-versa, and the issue stayed with the speaker cabinet. The knocking sound did not move with the woofer, correct?
In that case, it has to be something inside of the cabinet. Take out the woofer and look/feel for anything loose, like the wiring or a crossover component/board or an internal brace. Also check all of the screws in every driver and the crossover board to cabinet.
How heavily are you tapping on the dust cover? Would seem to me that if you tapped it hard enough to cause a knock, you would be denting the duct cover. Are you actually tapping the cone and not the dust cover?
So, you hear a knocking when you tap the dustcover, but your friend from Yamaha doesn't hear it? Do you hear an actual degradation when playing music? If not, forget about it and stop tapping the dustcover.
When you say "swapped woofers, same result" you mean you moved the woofer from the left speaker into the right speaker, and vice-versa, and the issue stayed with the speaker cabinet. The knocking sound did not move with the woofer, correct?
In that case, it has to be something inside of the cabinet. Take out the woofer and look/feel for anything loose, like the wiring or a crossover component/board or an internal brace. Also check all of the screws in every driver and the crossover board to cabinet.
How heavily are you tapping on the dust cover? Would seem to me that if you tapped it hard enough to cause a knock, you would be denting the duct cover. Are you actually tapping the cone and not the dust cover?
So, you hear a knocking when you tap the dustcover, but your friend from Yamaha doesn't hear it? Do you hear an actual degradation when playing music? If not, forget about it and stop tapping the dustcover.