There have been a lot of questions/concerns posted about the EVO's "whistle" . . .
The whistling is created by the EVO's air turbulence. In tests by VXL the higher pitch correlated to approximately maximum turbulence.
Some may recall discussion after the OG release about its airflow pattern dropping the pressure in the ELB and causing the air turbulence that moves through the ELB which makes for the unit's superior extraction. The redesigned bamboo in the EVO improves upon this, creating even more turbulence, and hence the wider range spectrum and even more efficient extraction - and the whistle. A contributing factor btw can be the diameter of the bamboo; the tolerance is tight but even within that close range a very small difference may affect the whistle.
As a user I can change the whistle by doing something that changes the airflow. One of my EVO's has a single high-pitched whistle that is only reached with a very strong pull. The other EVO has a soft low-pitch whistle if using a light draw, plus a high-pitched whistle reached with not quite as strong a pull as the other unit (so it can be jumped over). Ordinarily with both units I begin my draw relaxed and then quickly (but not sharply) accelerate the intake past the low-pitch whistle (if there) up to the just under the high-pitch whistle and sustain the draw there. I've found that at this pull or a bit less is the most comfortable and produces just as excellent vapor.
But note that other airflow factors may add or subtract from the turbulence whistle: My HydraLine and HydraCirc almost totally dampen the whistle, my Fanatic Custom Swagger also dampens it but less so, but my Mobius Ion Matrix with its greater airflow and the EVO inverted produces a very loud whistle (which I stay just under, which again feels optimum despite the difference). How full and dense the ELB load is and whether the ELB is fresh vs clogged, have also sometimes had a small but noticeable effect.
Some dampen the whistle by throttling the intake, with a finger or a makeshift baffle from tissue paper or foam. Personally speaking, with practice I can now pretty much anticipate the whistle with whatever I'm using, so slightly adjusting my draw technique is becoming intuitive. SM told us that "if you are inhaling below or above that speed (where the whistle is loudest), it tends to just go away". That's essentially been my experience, too.