Thank you for clarifying that. I always thought that sounded too realistic to be some synthetically produced noise, but, not being a musician, I didn’t know. That song brings me to a peaceful plane whenever I think about it, including the sound of the Cymbal being struck or brushed at the end of the measures.
Mellotron was an interesting instrument. It was basically bunch of loops of recording tape, each with a recording of a single note from one or more instruments. When you pressed the key a tape recorder head pressed down on the tape. There was a slight delay between pushing the key and the start of the sound, making it hard to play melodies. They were mainly used to add sweeping orchestral chords like those in the title song of the album.
Synthesizers had made it pretty much obsolete by the late 70s.