I'd use a combination of a low frequency sine wave, as well as another sine at the first harmonic of the initial sine wave.
distort the first harmonic enough to give it the proper crunch.
I'd use FM8 (my fm synth of choice) with 3 operators
first one (carrier) at the fundamental sine's frequency
second one (modulator) at a ratio of around .25, fully modulating the carrier, set to the "10th" formant waveform.
maybe add a 3rd op for subtle upper harmonics to the 2nd op. then just distort it, and get it layered nicely.
modulate the carrier with the 2nd op @ 100%.
bear in mind, this is without having the synth in front of me. I'm just kinda doing it in my head. this would make a nice rattle effect tho.