OK, I tried various cartos/clearos/tanks on both my Acrylimax (which uses the Smoktech Zmax chip) and the Vamp. Some types of coils cause audible stuttering, others don't. So, as Dirk states, the rattlesnake noise (which reminds me of an old Schwinn bicycle with a floppy playing card clothes-pinned to the spokes, more like a flutter than a machine gun) happens in both units "with certain devices."
Apologies for the incorrect info in my previous post. I was using a tank in the Vamo with a carto that didn't react and was/is silent. Interesting phenomenon. I'm not bothered by the flutter sound, but I can see how it might aggravate some people.
I don't think it's a massive problem, it certainly wouldn't bother me.