i have been struggling with the crackling sound for two months and I even demanded a new BPAP machine because I believed the mind-bending noise was originating with the fan in the motor. Finally I purchased two new air vent flaps (terrible design flaws) and compared them to my current. My current flap/vent was labeled #84. The two i purchased today were #87 and #88. Is this sizing? I use a medium mask. I did a test drive on the #87 at my provider's business and SHOCK it eliminated the crackling sound. Why doesn't the owner's manual reference these vent/flap sizing issues. My sleep really deteriorated during this two month journey. BUILD A BETTER PRODUCT!!
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Worst Innovation EVER
February 5, 2013
Reviewer:
Toni Carrell from California
Special note: This review is written incorrectly because we are not allowed to use any punctuation symbols except periods. Sorry.
This thing is so touchy and difficult to seat in mask properly I will NEVER buy another. I could not make it stop whistling. Eventually, after much frustration, I discovered that the thing was seated in the mask UPSIDE DOWN, exclamation point. Once I flipped it and wrestled with it for what seemed like FOREVER, I eventually got it seated correctly and it FINALLY stopped whistling. I really think this is a design flaw. There needs to be some sort of indicator, besides the tiny little bump, to let you know if it is upside down other than the annoying whistle. Better instructions that don't resemble Ikea diagrams would help, too. It also needs a deeper groove or wider flanges so it's easier to seat correctly. I only gave it one star because this site wouldn't allow me to leave ZERO STARS as it DESERVES.
WHISTLING/CRACKLING NOISE AT THE AIR VENT 
Worst Innovation EVER