I don't own a ModFactor, but from your description, it sounds like classic through-zero flanging. There's a very small, fixed delay time (a few milliseconds); alongside a delay time modulated by an LFO. When the LFO-modulated delay time falls below the fixed delay time, you'll hear that 'sucked-through-a-vacuum' sound.
When the LFO bottoms out, it starts out the sweep at a shorter delay time than the fixed delay. As it reaches the fixed delay time, it hits the null again on its way back up. It makes sense to me that you'd hear that with a sine / triangle modulation, but not with a ramp/peak modulation (due to their characteristic 'shapes').