Knocking sound from mask/machine?

When I did my awful sleeptest I was woken several times by an awful knocking sound. The sleep tech put it down to humidity problems.

The other night the same noise happened on my machine at home the minute I put it on.
There was no humidity and no other problems that I could see.
It only happened on exhaling and seemed to stop after a while.
Was not on every exhale.
Any ideas what causes this?

When that happens to me I take my mask off and hang it down because there is usually water sitting in my hose. So I have to drain it, sort of. Then the noise stops.

Ah, that makes sense. How would the water have accumulated if I hadn't used it since the night before?

Humid air condenses and forms water inside your hose. If enough collects, it pools in the low spot in the hose and blocks off the airflow. As air bubbles past the water blockage, it makes weird noises.

Many people are bothered by "rainout." Solutions include turning the humidifier down, arranging the hose so water flows back into the machine or figuring out how to keep the hose warm. There are heated hoses, hose covers, putting the hose under your bed covers, or turning up the temperature in the room.