It is quite possible that what you are hearing is the voice coil scraping against the magnet structure surrounding it.

It only takes a very slight distortion or misalignment of the cone to cause this, as the clearance is very small between them.

The only satisfactory way to repair this is to buy a new replacement woofer from KEF and install it. It is almost impossible to do a repair that will restore the alignment properly.

forty years ago KEF made what audiophiles at the time considered one of the best sounding speakers in the world, the KEF 105.
I listened to them in a showroom, and they were amazing. They beat out the Dahlquist DQ-10 and were the equal of the Vandersteens and even the Quad electrostatic.
Don't know if Quad's current speakers are as good.
At that time my buddy had Acoustat Monitors, an electrostatic that was also incredible sounding.

shag57 wrote:

So I sent my 4 woofers and the midrange / tweeter unit/component back to KEF to get the tweeters replaced and woofers reconed. They quoted me a price. When they were testing them after the work they discovered one of the mid range speakers was starting to separate. They refurbished all four midrange speakers and didn't charge me. I've had them back for several weeks (50 hours play time) and I can say they did a fantastic job. They sound great. Very impressed with KEF's customer service and commitment to customer satisfaction.

That is great to hear about. So many people have bad experiences with getting things serviced (or can't get any replacements at all), so it is good to know that KEF is so good to their customers.