Yup, the toilets are always one of the things the first comedian act of the cruise hits on. If you have ever flown and used the bathroom on the airplane, its very simliar. When flushed, the vaccum makes a large whoosing sound and almost a pop. The big joke is that if you are a big person, never hit the flush button while sitting down or they will have to come to your cabin and pry you from the seat. (Either that our you will walk away with a very large hicky where you don't want one.
In reality its not really that bad, but it does take the unsuspecting person by surprise.
My travel agent told us not to be sitting when we flushed. Boy oh boy does it take you by surprise. When we told my cousin about it on our cruise in August, she didnt believe us. She had never been on a cruise before. I told her to give it a try but even though she didnt believe us, she didnt try it either.
Oh yes, definetly close the lid... otherwise the suction will pull in the bath towels and soap.... and maybe even the shower curtain!!
Just joking of course... but Dave makes a wise point... besides, they purposely put the flush button on the wall behind the toilet so that you have to put the lid at least half way down to flush anyway. I assumed that was "by design" and a important -anti sloshing mechanism..
Those things are wild. My friends were with us for their second cruise this time and came by our cabin the last morning to wait. She used the bathroom and came out shaking her head. She had been sitting on the rim all week thinking the other part was part of the lid