PWM fans are generally great, but kidsafe has a very good point. A lot of people can hear a slight "ticking" sound that's associated with PWM fans (I'm guessing its the control signal measuring RPMs?). However, I think generally, PWM fans run a bit quieter than a lot of the other fans out there, and they can "auto adjust" to suit the current need of the case environment so that they're spinning even slower when appropriate.
That being said, there's a lot of 3-pin / Case fans out there that are just as good (if not better) for quietness. SPCR is a big fan of the Nexus Real Silent 120mm for instance, and so am I
Pair a a nice quiet 3 pin fan up with some sort of voltage regulation (as you noted) and you've also got a pretty good deal going.
Either way you go, if you're interested in building a quiet rig, I'd almost always replace the stock fans as a rule of thumb:
- Fans that come with stock heat sinks (AMD/Intel) always should be replaced, along with the heat sink
- Fans that ship as part of a 3rd party heat sink can usually be replaced to some benefit
- Fans that come stock with cases generally can be replaced with something better
In your case of the tricool, I'd definitely consider getting a quieter case fan. They're not terrible, but you can do a lot better on the quiet front.