I just picked up an Electro Harmonix Mel9 effects box. It mimics the Mellotron keyboard sounds of the '60s and '70s (strings, choir, flute, etc.) This was done using the cello sound.
The box also appears to pick up the noise of strumming the strings. You can hear strange bass sounds, and I think that's what is causing them. Using some of the Mel9 sounds, it sounds almost like a tympani. On the choir sounds, it sounds like the Wicked Witch of the West's guards singing bass notes.