What amplifier are you running them on, the name of that amplifier the impedance of the amplifier and the wattage of the amplifier.
Also what is the impedance of the loudspeakers?
Are you wiring the loudspeakers up to one terminal on the amplifier, as I have done this myself before, and I will, never repeat this exercise as this will put strain on the circuitry and the loudspeakers as well?
When you turn up the volume to a "certain point", you have exceeded the limits of the driver's motor with that particular signal. The voice coil is being pushed out of the magnetic gap between the pole piece in the center of the system and the magnet structure itself on the outside. When the voice coil is pushed outside of this area, the travel of the coil cannot remain linear and so it distorts to wherever the cone drags it. When it inevitably has to return to a resting point inside the magnetic gap, the moving edge of the voice coil catches on the fixed edge of the gap. If you continue to do this there will eventually be a deformity in the voice coil that catches over and over again. This could be what you are already hearing. You can either get the driver repaired or you can continue to turn up the volume to that "certain point" and the noise will eventually go away. Of, course, at that point, so will all the sound from the driver as it will have been destroyed.