Why do my ears pop in an aeroplane when it is climbing and descending?
[The air pressure in the cabin is changing.]
From: Bill Sherwood
Yep, that's about it. the general rule is that we try to make the cabin altitude go up & down at no more than 500' per minute, as this is a safe threshold for most people so their ears don't cause trouble. One trick that a lot of pilots use is to hold your hose with your fingers and blow, thus clearing your ears. You'll hear a bit of a pop, then the pressure will equalise.