Why does the mask valve sometime 'thump'?

[StevesSp]

Seems it might indicate low flow from the machine, which in turn suggests a fault, given it's happening after hours of normal operation?

A few months ago, the machine started behaving strangely, 'waking up' whenever I moved the hose even slightly. The supplier said thay all do that, which I know isn't true but, because the machine otherwise seemed OK, I forgot about it. Now this.

[pholynyk]

My thought was that a strong exhale might exceed the venting of the mask, if the vent was a bit clogged. This would reduce the inflow from the machine, possibly even reversing it. Then when the exhale ended the inflow would snap the AAV closed, resulting in the thump.

Just a guess on my part; the only ffm I used has a soft plastic flap that couldn't possibly thump.