I was milling around on freesounds.org
out of sheer curiosity (by the way, any producers reading this, I totally recommend it for quirky sound effects!), and I found this interesting sample of Looney Tunes-esque 'boink' sound effects played one after the other.
Then, a lightbulb went off in my brain and I had a "what if I made something with this?". Some experimentation with effects later, I had the sound you hear in this track!