Piano VSTs with weird high overtone feedback

That's the string sample, if the show the same string used for the same few notes that irritate you then try to ditch that string sample and replace it with the next string above or below it. It might be very easy compared to trying to filter it. Once you swap it you can confirm you did it right by playing it. The pitch of the note will be the same as the note you got the sample from. Then all you need to do is tune the sample and that's it. You kind of have to learn something about the note though. Sometimes there a two samples per note. One sample is recorded with the sustain pedal down and the other is with the sustain pedal up. So you may have to learn how to deal with that. Also each string is generally sample many times at different key velocities. This means there can be a lot of samples for each note. So it you use a different string you have to move all of the samples along with it. Most likely you have to highlight each sample, copy that and paste it where the bad string was removed. Do you know how to get into the Kontakt forum. They'll know exactly what to do and it'll be a breeze.

There's probably a lot on you tube and the kontakt forum can get you going. The owners manual for Kontakt is probably excellent. I'd watch you tube stuff to get the general idea then read the manual and ask questions in the forum. Type Kontact on your question and at the bottom and it should get you in there. Have fun, yeah samplers are cool, they're actually very simple, the hard part is getting the samples. I was going to make one with my Coronet someday.