Welcome to the world of hyperrealism! What you hear is the imitation of the damper hitting the strings when you release the pedal. Watch at the display, this key off dampler sound is introduced on key off actions. You can turn it off, mute the element or the part.

If it is happening “with high volume” then you have edited the Element responsible, usually it is mixed in at a lower volume as this is the subtle sound of the hammers falling back and felt falling back on the string. Which Performance has it sound “with high volume”?

XA CONTROL (if you want to read about in the Reference Manual) Set to ”Key Off” sound... which engages very time you release a key or release the Sustain pedal, same as would occur on an acoustic piano. XA CONTROL is found by editing the Part, and selecting each Element in turn.

You don’t mention a specific Performance... but usually the last Element is programmed to do KEY OFF in acoustic piano Performances. XA CONTROL is found on the Element > OSC/Wave screen

Thanks a lot, it was the XA CONTROL on KEY OFF indeed.
For me, it's sounds way to high and not musical. (again, if you hit the keys hard and wait for total fade and only then let go).
It happens on the default CFX performances, so it's not something I changed.

I fixed it by lowering the specific part level.

Thanks for your help.

Using the preset "CFX Concert" - the factory preset, not anything modified - I hear exactly what you mean.

First, strike (and hold) middle C at a mf - nothing too crazy loud so it doesn't take "forever" for the note to decay to silence. This is holding a key down - I'm not using the sustain pedal or anything. So you hear "DAAAAAAAaaaaaaa ........ (silence)" and at the silence - the key is still held down.

Next, lift the key in silence. You'll hear a middle "C" pop out "bip". A soft staccato note. This isn't something I regard as a natural thing. When the damper felt lays back down on a string that's not moving - it doesn't "strike" a note back. At least not as noticeably as is happening here. The time I do hear this sort of thing is when the a damper felt "pulls off" the string - and a string will ring even if not struck by the hammer. The other way - the felt laying on a non-vibrating string I cannot say I would expect this kind of a sound.

In order to really hear this, it helps to put volume pegged at maximum. Volume at 75% is still very noticeable. Volume at 50% and it is very hard for me to notice if I'm not focused on this one item.

I haven't mic'd up a CFX to amplify the instrument so you could do an equivalent of turning up Montage's CFX to 100% volume. I'm not sure if you did this if you would hear the same kind of "bip". Maybe you would and the recreation is perfect. Not expected, certainly - but maybe it's there.

Maybe there's something you can do with envelope follower that would leave the key release XA control at full volume if the envelope follower detects the piano strings are sounding (vibrating) - but will reduce the key release volume if the envelope follower does not see any sound. This might allow for more realistic key release noises before the strings decay down to silence - but remove the sound in silence. This would allow for just the unwanted sound to be surgically removed without affecting the sound substantially before the decay.