It would be impossible to diagnose your issue from this vantage point. If indeed the "thump" is caused by the felt being somehow "dry and brittle" then it could be an indication of a more serious problem... It's just impossible to tell without actually seeing it. Your description is simply not enough information to tell you anything. I strongly suggest you take the instrument to a qualified authorized service center. Or you can call a customer service technician and talk with them over the phone. I'm sure they will have questions concerning the history of your instrument, where its kept, where or not the sound is audible acoustically or through the sound system (there are "thumps" actually modeled in the technology - and many people mistake this for a key "problem" when actually there is a programmable amount of "thump" as apart of the model... They will make sure you can tell that difference BEFORE they send you off to get a qualified second opinion.

If you are in the USA please call the customer support folks first.

Thanks for your response, what I neglected to say was " its a mechanical noise " The keys make that noise even when the keyboard power is turned off. I am going to contact customer support.

I had that exact issue with my CP33, and came up with a solution that was simple and effective. I took some red piano felt and cut a 1/2" strip of it as long as the keyboard. I tucked that strip underneath the existing felt strip (i.e., between the keytops and the existing felt), lining it up neatly with the existing felt. At this point you can test and see if it solves the problem. It did for me, so I then glued the new strip in place to the old strip, inserting a tiny dab of carpenter's glue, using a toothpick, every few inches. It doesn't take much to hold it in place, because the keys are already pressing it to the existing felt strip. For me, this was a 100% solution. YMMV. This solution is also completely reversible. Good luck!

Thanks for the advice, I will give it a try.

I would be grateful for information regarding where to purchasing the piano felt you used to resolve your instrument problem. I have tried a number of stores without success. Thanks

Actually, my piano tuner gave it to me (I also have an acoustic piano). It's just thin wool felt, which you might even be able to get from a fabric store. Or here's some online: http://everythingforyourpiano.com/shop/felt-cloth/piano-nameboard-felt-medium/

Here's a photo ... you can just see the edge of the added felt (it's slightly redder) under the existing felt ...

Thanks for the information, I have ordered the felt material from the website you indicated, and from the picture you sent, I now know exactly where it is to be placed. Thanks again.