Cool sound!
Maybe... take a mono LFO, set it to saw ramp down, assign it filter cutoff at max. Turn the filter cutoff all the way down.
Set the LFO to re-trigger on keypress, that way it always starts at the same place.
Then assign a spare envelope to the LFO speed. Set the decay up and the sustain at zero. That way it starts fast, then slowly decays down to whatever the speed of the LFO is. Then play around with the LFO speed, envelope decay, and env to speed depth.
The above example works well on the Take-5/TEO-5, but can map to any synth.