I have an iPad Air and I do notice a bit more of a vibration than with the iPad 4. My iPad isn't in a full case yet (my store hadn't gotten them in yet), so I don't know if that will make a difference or not. I did do a test with my iPad Air and iPad 4 - I took the case off my 4 and played the same You Tube video on both devices at the same time at the same volume. The Air vibrated more than the 4, but the 4 was not without vibration entirely. I think its because its thinner, so the iPad Air doesn't absorb the sound as much as the thicker iPad 4. There's less space between the speaker and the iPads backing, so you feel more of what's going on inside, whereas the previous iPads were so much thicker, you didn't feel its "insides" as much (does that make sense?) I also think because the bevel is smaller on the Air, the sound is more contained - for lack of a better word - whereas the older iPads have wider frames, so the sound is distributed more widely. Does that make sense? Between its thinness and smaller frame, its only natural you'll feel more of a vibration.
It wasn't enough to be bothersome (to be honest, I never really noticed until you pointed it out & asked about it!) and I do wonder whether or not a case will make a difference.