I've been looking online for instructions to self-repair a Resmed S9 without luck, so tonight I tore mine apart because it was making so much noise I couldn't use it, anyway. It turns out it was the humidity sensor airflow tunnel (I think) that was causing the noise. Anyway, here are the instructions:
*** You will need a Torx 10 screwdriver for disassembly. Do not use a phillips screwdriver, you will damage the screws. It must be Torx 10. You will also need a flathead screwdriver and possibly two smaller screwdrivers as well. ***
1) Remove the power cord (duh!) and the SD card, as well as the air filter cover. Pry up the control knob and remove:
2) There is only one screw on the outer casing you need to remove before taking the S9 apart:
3) The top and bottom plastic covers are held on with plastic clips on the inside. You can use a screwdriver to gently pry each half off, moving along the edge two inches or so at a time:
There are two particularly stubborn tabs you can reach from the outside with a small flathead screwdriver to remove the top. You can see the fruits of my bad screwdriver work before I realized how to release those tabs:
4) Once the two covers are off, you need to remove the circuit board. Ground yourself! Static electricity can damage the board! There are two screws that hold the board on, and a clip for the power jack that must be unclipped in step 5. Remove the two screws first:
5) In order to lift the circuit board off the frame, you need to unclip the power jack. The clip is accessible from the outside of the housing. You'll need to use two implements (I used two small screwdrivers) to pinch the feet of the clip sideways towards each other to release it.
6) Gently lift off the circuit board since the power plug for the motor is still attached. There are a few items to note: The power cord for the motor, the CMOS battery (if your CPAP loses time while unplugged, you'll need to replace this), the two black plastic ports that line up with the two holes on the base (for the humidity sensor) and the small gap at the front of the housing that the motor wire runs through. Take note of that gap - you can't get the covers back on unless the wire is placed back in there. Remove the power cord for the motor from the circuit board.
7) You'll have to trust me on this one. Even though you can't see them, there are five Torx 10 screws that hold the motor housing to the chassis. Follow the arrows and remove the five screws:
8 ) Disassembly is mostly complete. The fan unit is housed in a rubber shell which is keyed with inserts so you can't put the blower in the wrong way. I was looking for a part number in case I had to replace the blower, but there was none to be found. The plastic case that shrouds the motor and fan assembly is glued together and can't be taken apart without breaking it. In my case, the air redirector for the humidity sensor had dirt in it (the light brown tube in the picture), which made it whistle when the unit was on. I blew that out and the fan blades with some compressed air and the noise went away after re-assembly.
Putting it back together is simply reversing the steps. The plastic clips on the outer covers are particularly stiff, so you'll need some pressure to get them back on. On the top cover there are two clips that mount on the circuit board - be gentle when pushing those back into place.
I hope someone can use this. I could not find a guide online for the S9 anywhere that included instructions for taking it apart.