Are your speaker woofer cones "poly" material? If so, they are doing what all poly cones do when driven hard they produce a 'snap' or 'crack' sound. This is normal for a poly (plastic) cone.
One of the drawbacks of a poly cone material.
If your woofer cones are some non-plastic material, I do not know what is wrong.
One of the drawbacks of a poly cone material.
If your woofer cones are some non-plastic material, I do not know what is wrong.