how to build this sound (mug bass)

starting at 1:39, there's scrubby FM bassline whose notes don't seem to have a distinct attack. I guess it's portamento, but if anybody has an idea how to build this sound, please serve up yo' best guess. it fascinates me and of course i'd like to know a little about how it works.

It sounds like the cutoff & resonance are sequenced with some kind of 303-like step sequencer, with variable glide and such. Or someone just very nimbly manning those two knobs on the filter. It doesn't sound like something you can do with most synths in the box because of the difference in attacks & decays of the filter envelope.

My .02

im a firm believer that anything that can come out of the box can be made in the box, given enough time.