I believe i'm pretty quiet while riding. In fact, i think I've never made a sound in a coaster (apart from one or two comments that I may have made during pauses like MCBR).
As to the "no hands" thing: when I was younger I used to think that it was childish too. However, nowadays, I do it all the time. I don't really know why, it´s kind of automatic, not to show off that's for sure. For instance, there was one moment when I was riding furius baco in which I decided to hold the restrains so that, by the end of the ride, they wouldn't be crushing my legs as it usually happens (it happened to me while riding kingda ka too). Despite this, right after the launch, I just had some sort of impulse and raised my hands as inevitable. It was a bit strange but I really wanted to have my hands in the air for my own pleasure. So, it's not necessarily to show off.