Those are higher end rigs, either high frequency PWM or direct DC with AC overlay...the Vamo and other more "inexpensive" AVP's that do VW hit at 33.3hz which give that slow hiss or sizzle...its just the result of the on/off cycles of the DC pulse to give the RMS equivalent of the voltage to the coilI don't get the "rattlesnake" sound from any of my mods ... Joyetech eVic, Sigelei Legend, Innokin iTaste VTR, iTaste VV, and hopefully not from my MVP 2 when it arrives ...
I have also vaped with a DNA20 mod and not heard it there either (but I don't actually own one ... yet).
As always ... YMMV!
HTH
The rattlesnake shake is a powerful symbol in ancient Chinese lore. Embrace it and enjoy...