So I am sat in row 30 on a Pegasus Airlines 738 the other week, flying back from sunny Turkey to rainy Luton. Boo Hoo.
Anyway, on climb out, there is a very audible hissing sound, that increases in sound as we are going up - almost like an air escape that gets greater as we gain altitude. I thought it might be aerodynamic - but it disappeared by the time we were at cruise. Then I thought it may have been the cabin pressurising, and the noise was the outflow - or even the door seals expanding and sealing the escaping air.
Any ideas anyone?