That's weird about that string or organ background for the piano sounds! I just checked those sounds on my Mellotron Rack and Micro and it doesn't sound like that. hhhmmm, was your A/B mix control not fully set to just A and a little of the Open Trumpet on B leaking through? 🤔 If not you might want to write to Marcus at Mellotron and point him to this video to see if he thinks you might have a defective unit?