Whilst I can understand the desire for passengers to want to know the source of every strange noise heard on board an aircraft, unless there is a safety implication, then I see no reason for the pilots to make a PA. The exact nature and cause of the noise is not always obvious and so it becomes a matter of speculation, and I'm not in favour of providing passengers with spurious explanations based on guesswork. It can be more unhelpful than helpful. Take the example that made the papers a few years ago of the pilot "not qualified to land in fog"; a pilot trying to offer a well-meaning explanation to passengers for a weather diversion in terms they might understand and instead creating the opportunity for some mischief-maker to get their name in the papers with some twisted and sensational headline.
In way of an explanation for the original poster, I agree with those who point the finger at a door seal. These tend to produce high-pitched whistling or whining noises, whereas the noise suppression system has a tendency to either elevate the general level of engine noise in the cabin, or produce a knocking sound in the area of failed actuators.
Neither of these situations have an immediate safety implication, particularly if all cabin pressurisation indications are normal.