While watching a Mil Mi-17 helicopter takeoff I experienced strange thing. The helicopter produces a lot of noise even when just starting the engines.
But shortly before taking off the high-pitched sound of the jet engines changed somehow (I guess changing the tilt of the rotor blades) and I felt some sort o pulsing "sound" which I felt rather with my belly than with my ears. Really unpleasant experience.
Could it be infrasound?