Yamaha P-70 digital piano. A few keys are making a loud knocking noise. The note still sounds, but the knocking noise on the few keys can be heard at the other end of the house

SOURCE: Yamaha digital piano clp 170

This is a common problem with Yamaha keyboards. The problem is caused by worn-out rubber contacts in the keyboard assembly. In my old Clavinova CLP-500, there was one long rubber contact strip under the keys with two parallel strips of semiconductor material. The Clavinova circuitry determines key velocity by measuring the timing between when the key hits the first and second strips of semiconductor material. The harder you play a key, the less time it takes the key to hit the second semiconductor strip after hitting the first.
After years of playing the keyboard, the keys eventually cause tears in the semiconductor material and this messes up the timing measurement for key velocity. The only solution is to take the keyboard apart and replace the rubber contact strip with a new one.
I got rid of my CLP-500 four or five years ago and got a new CLP-170. The CLP-170 is now having exactly the same problem that the CLP-500 had. Yamaha has re-designed the rubber contacts in the CLP-170 so that there are now eight individual contact strips instead of one long one. The problem is essentially the same, though. You have to take the keyboard apart and replace the worn out rubber contacts.
Here are the part numbers for the rubber contacts that need to be replaced in the CLP-170: V8286600 Rubber Contact, 12 keys, D-C#   Qty. 6 V8286800 Rubber Contact, 11 keys, A-C#   Qty. 1 V8286700 Rubber Contact, 5 keys, D-C   Qty. 1
I suggest you also get a copy of the CLP-170 service manual, part number 001677. It has descriptions of all the steps necessary to take the thing apart and put it back together again. You'll also need a "rod" (which is just a long dowel), part number TX000670. Before you take the circuit boards off the keyboard assembly, you have to insert the dowel between the keys and the frame to keep the keys from falling back and getting in your way. A 5-foot long 1/4-inch dowel will probably work. (I haven't yet taken apart my CLP-170, and I don't have Yamaha's "rod," so I'm not sure if 1/4 inch is the right size or not. I'll report back here if it's not.)
Good luck, Howard

SOURCE: Sticky Keys for Yamaha Digital Piano YDP-121

have you tried using the can of air. you can get it at almost any drug store walmart or kmart. just spray the can of air between the keys and they should become unstuck.

SOURCE: Yamaha CLP-860 Digital Piano Keys Making Noises

The part numbers are only available through the Yamaha Corporation. The Service manual, which is available for purchse (note that this is NOT the owner's manual) will have them.

I repaired mine on my own. I used a sticky foam tape of the correct width (approxiately 3/8 wide) and overlaid one layer directly over the original felt pad. I used a white foam tape like is used for carpet or other indoor needs. If I were doing it again I would probably use something just a little thicker and softer than what I used (appx. 1/16 thick).

Disassembling the Clavinova and removing the entire GH88 keyboard for this work is not hard if you are even moderately handy. I had the entire mechanics of the unit laid bare in 45 minutes with no manual (and not one curse word).

I am going to write up a tutorial on this soon as many people have asked about it.

By th way, I have heard that the factory pads are $100. My solution cost me about $3. Even if I have to do it again in a year, it's a win.