phantom tremolo out of key trill in playback

yesterday, 3/27/2025, I installed the new update. I am running version 4.5.1.250800846 . I went to make minor adjustments to a finished big band score (attached). In playback those bars (measures 69-74) there is a weird phantom tremolo, out of key trill that sounds like an organ. It is not in any part and I muted the chords to guitar, piano and bass. What is it and how do I get rid of it. Unrelated ... Is there no way to for the midi to "play 2nd X only'?
Thanks!! Frank Southecorvo