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Source of Resmed S9 EPR reverse flow whistle!
I've had this unit probably 3 years and experienced the oh-so-common whistle for the past 2.

My cpap is inside my night stand. I put a 2" hole in the back for electricity, inside light and the extra long hose. I run the hose up over the back of my headboard.

This install hides the ugly unit (cuz it's so cool for a fit athletic guy in 30s to have a cpap), hides the distilled gals of water), keeps the unit cleaner, hides 90% of the cpap noise, and most important, keeps me from rolling over on the hose!

The last week of so, the d**n whistle has bothered me a lot when trying to fall to sleep. I decided I would see if I could narrow this down... tried all the component, removed the filter, removed humidifier, different hose.... always have the whistle exactly the same. While the cpap was running at a 9.3, I put my hand over the output where the hose goes on the side (humidifier removed) and THERE IT WAS! The whistle! Nonstop!

I love to take stuff apart and see how it works. But I don't want to risk it o this life changing machine.

Has anyone taken the machine apart and run it with the inside exposed? Narrowed down anymore where the whistle is from? It seems to me, it's the backpressure causing it. Perhaps there is a check valve that has some rubber that vibrates? Perhaps part of the check/resistance valve is torn?

It just seems, this whistle is made by a vibrating piece of rubber - like a whoppie cushion. If someone would prove this and find a modification, you'd have a ton of happy cpap users!!!!
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Hi neal,
WELCOME! to the forum.!
Good luck to you with your CPAP therapy and finding out where the whistle is in your machine.
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Probably not far off. Rubber seals degrade over time. I would look at one of those.

Using FlashAir W-03 SD card in machine. Access through wifi with FlashPAP or Sleep Master utilities.

I wanted to learn Binary so I enrolled in Binary 101. I seemed to have missed the first four courses. Big Grinnie

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Thanks for the welcome.

Do any schematics exist for the Resmed S9 to anyone's knowledge,
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No. They consider them trade secret.

Admin Note:
JustMongo passed away in August 2017
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~ Rest in Peace ~
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the 3 colored wires connecting the motor to the computer board are fed though the silicon seal between the top and bottom of the motor compartment at the front of the machine. you can find pictures under s9 teardown on google, I think.
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