I fly Dash8-300's and not Q400's, but they both have an ANVS (Active Noise and Vibration Suppression) system.
Occasionally they don't work (or don't work in certain parts of the cabin) but the sound described doesn't sound like a faulty ANVS.
In my airline (Aussie regional) there is no mandated PA that must be given in the situation described.
Perhaps there should have been some kind of PA, but if the crew didn't know what the cause of the sound was, they're not likely to make a PA saying "We don't know what it is", and the same would apply if they suspected that it was a dodgy seal letting air out.
My thoughts,
DIVOSH!
Last edited by Di_Vosh; 19th Apr 2012 at 06:21.