All L4s make a slight click in the head when activated (as mentioned in an earlier thread). But if you hear more of a hiss and from the tail, then it is probably a 'dirty' switch. Mind you, the regulation circuit may hide the problem by not flickering the light (for the dirty switch, not talking about dropping out of regulation). My Z57 switch does not flicker in momentary mode on a two cell body. On an e1e+KL4 it can flicker a bit. My guess is that the regulation is hiding the dirty power, but the TW4 config is never in regulation mode.
I too am still on my first battery set, so I am no expert (not yet anyway). I keep switching back and forth between L4 and TW4 configurations. I should probably just get a second L4 so that I have two KL4 heads, but money is tight right now.
Erik.