I believe WHBM's father has it.
It is probably due to the aircraft flying out of ground effect. As the aircraft flies to a height greater than it's semi span above the ground it experiences a slight reduction in lift and hence the wings fully taking the weight of the aircraft. People near the CoG won't notice it as much as people at the extremes as the sensation is exacerbated by fuselage bending about the CoG and hence the tail and nose will dip, the tail dipping more than the nose (as it has a greater distance from the CoG).