I noticed that on the A320s there are a lot of airframe vibrations and shudders from the start of rotation to the climb attitude on takeoff. It can only be felt at the front to the mid cabin area. Is this simply the result of "introducing" the airframe to the airflow or are there any other factors? I'm not sure if this is common on other aircraft types; I've only had the chance to be in the front on the A320s.