Interesting that you're considering that lens. I just bought myself the 18-135 and I'm pretty happy with it so far! Quiet, much more versatile as a solo walk-around lens, and it'll still be useful if I decide to upgrade to a Pentax DSLR in the future.
The AF on my copy is a little louder than I expected it to be, though. I know it's a K-01 and has the slower CDAF, but here's a video showing what I mean. It was in a darkish environment and I'd have to test it side by side with a DSLR to be sure, but let me know what you guys think.
Edit: I directly compared it to my 40mm XS lens and WOW. No, the 18-135mm isn't completely silent, but MUCH quieter than the kit lens and in the real world (compared to my unnaturally quiet room), the sound is basically imperceptible.