It is possible that the voice coil of your woofer may have shifted a little and is touching the center magnet pole. With the amp off, try pushing the woofer cone with even pressure and see if you feel it is touching something.
It is also possible that the wire inside the speaker may have shifted and is touching the woofer cone. In this case, you will need to take the woofer off and look inside.
It is also possible that after years of service, the suspension of the woofer get lose and the voice coil hits the bottom of the magnet when playing music with heavy transients. You might want to look at how the woofer moves when music is playing.
Another possibility is that your speakers are fine but there is something wrong with your amp.
Good luck.
It is also possible that the wire inside the speaker may have shifted and is touching the woofer cone. In this case, you will need to take the woofer off and look inside.
It is also possible that after years of service, the suspension of the woofer get lose and the voice coil hits the bottom of the magnet when playing music with heavy transients. You might want to look at how the woofer moves when music is playing.
Another possibility is that your speakers are fine but there is something wrong with your amp.
Good luck.