The first my dad knew something was up was the Cabin Crew started to look concerned, looking at one of the cabin doors. They were on the phone a lot and then one of them started banging on the cockpit looking quite alarmed.
Usually just a problem with the seal. Although the noise can be quite loud and disconcerting with the CC can be understandably concerned. But if the pressurization is normal (even if the door light is illuminated!), then all is well.