A Steinway D grand that I tune regularly has a noticeable knock when the sostenuto pedal is pressed. Two Steinway technicians who service the piano have denied its existence. Any ideas?

Check the rod mounting brackets for missing or damaged bushings.
Or look at or feel the damper blocks to see if any are being bumped when the rod rotates.

Greetings,
I would agree, if the knock is heard as you depress the pedal, with no keys touched,and you hear a knock, there is a damper tab that is being grazed by the sos. blade, or,the blade could be catching the edge of the back check on the the very ends of the rod.

If there are a couple of tabs being grazed, and they are adustable, papering them down will remedy the problem. That assumes the underlevers are hanging at the right height. It might be loose bushings in the rod brackets, it could be something in the trap work.
Good luck,

Possibly a keyframe bedding problem. When the pedal is depressed the keybed can flex downward, creating a gap between the glider bolts and the keybed.

Re. Steinway sostenuto knock.

Thanks for all the advice. I've been to the piano today. After having been sure in the past the rail was not catching any lips, today one was jumping up.

Having adjusted the rail to stop this, the knock has gone!

I feel a bit dim not to have realised the rail could knock without visual signs.

The day you stop learning something is the day to do something else!

Thanks again.

Ben Nolan