This is really quite simple. It sounds like a stretched sample, but it's actually just looped, with the modulator and delay setting on dtblkfx that is causing the change. Seems to work very well on short samples of around one or two seconds. Not much on the high end due to the resample effect. Anyway, it's easier to explain with pictures. Modulation isn't something I understand entirely, I just go by my ears. Oh, and there's comb and reverb on top of it.
Sample used here
Download if you wish here: Wave