I don't think drumming involves the wings, because the toms drumming I've seen aren't moving their wings when they drum. To me, the drum sounds like a low frequency (that is, a low pitched) "whoouump" or "ruuummp". It is a single note at a time. I think of it like hitting a tympani or bass drum once, only the sound has less volume and is lower in pitch than the usual tympani or bass drum. My understanding is that it comes from their chest area, likely the pulmonic membrane/sac. The "spit" is a "chhhkkk' sound I think they make when they "snap" their wings at the start of the strut, but I don't know if it comes from their mouths or their wings.
It is awesome to hear!!