Could be air escaping through your mouth or lips. Other problem is side sleeping will sometimes knock the mask. Sleeping on your back is worst for most as it is inclined to cause the throat to close and air will try to escape through the mouth or lips. Chin strap (a good one) or a cushion for the neck will help keep the mouth shut, but will not stop air escaping through the lips.
Sometimes the nasal pillows themselves can make a popping noise depending how you are breathing, this is not serious, but it does disturb the sleeper. If you change the way you are breathing it will stop instantly, but will return if you go back to breathing the same way. I get this sometimes, it is annoying, but breathing deeply get rid of it. Or just lift the pillows away slightly and try again, but if your breathing settles back to the same pattern it will return. It can be quite annoying, trouble is, when the pressure is low you do not feel the pillows moving, they just flex slightly with a popping noise.
I think the leak may have been from or through your mouth. Try a good chin strap or a neck cushion to keep your mouth closed. However, if air is escaping through your lips it is more difficult to stop. You could try taping then shut to see if this helps, but be careful what tape you use for obvious reasons,
How did it get wet? Was machine turned off with humidifier still attached (though you would think it should survive this if the board was treated). Did you knock it over with the humidifier attached? Did you pack it with some water still in the humidifier? Just food for thought. One thing though, did it get wet while in transit? If it did you may have a claim against the shipping agent? Overfilling the humidifier is another problem, but it needs to be well overfilled and it usually runs out the back of the humidifier jug before you can get to that point. Did you get the machine returned to you, without it you may not have a case to take to court. I would ask ResMed for a detailed report as to how they think it was damaged by water.
You can extend or make your own headgear. Make it so it can be adjusted with Velcro. The Swift FX headgear might be an option for over the head (but you might as well use Swift FX then, but the rear strap might be to small anyway, but make one and make it long enough with the soft Velcro covering most of the strap and two bits either end to attach it at the right length. It might be a bit tight to get through the loops, but with a bit of care it should fit and if you make it so as it is thinnest with no Velcro at the loops it should work fine. Or do as "Bonjour" said, just add a bit with Velcro to give it more length.
Francesco: With the small amount of CAs you are having I wouldn't worry much about it. Getting the lowest numbers is not always the best for yourself. It is how you feel, there is no point in getting a low AHI if you feel worse, sometimes getting a low number affect your sleep quality. If it is quite low, you feel good, do not fret about getting it lower. Sometimes getting it very low results in CAs increasing, it is a catch 22. If your AHI is below 5 and you feel good and not sleepy during the day, accept you feel fine and stop worrying.
Alcohol and eating late does not help, alcohol relaxes the muscles and can lead to your throat closing. Vellacook, I suspect you caught S.A. early. I have seen me stop breathing through the day when not thinking of anything but relaxed. Not a problem as I realise I have done this and my brain suddenly realises I am not breathing and jolts my brain into taking action. I have also felt my throat offer to close when I am sitting staring at the TV not interested in what is on and very relaxed (but not falling asleep), but again my brain kicks in and makes me do something about it, like sit up and pay attention!
Just another voice to add.
I knew a few people that were told a CPAP machine would NOT help them. This was poor advice and I got them on a trial, it helped the first night with some of them, (getting used to the machine is always a problem for some) so they bought machines (AUTO) and bought masks (which can be the most expensive part) and all but one person found it helped them greatly. The other I went to their doctor with X, the doctor sent X to have a dental mouth MAD device fitted to help X, I don't know if this helped X but it is worth a try if CPAP does not help, in most cases it will.