If you listen properly, then you'll hear that they definitely heard something from the outside. (maybe feedback from the studio, not necessarily BBLB crowd). Noirin says something like "did you hear that?", and Siavash say "Freddie must have lots of fans". Also look at Karly's facial expression
You then might get an idea why the penny has finally dropped..
http://www.channel4.com/bigbrother/v...fc9d85/play.c4