While driving about twenty mph I slowly let off of the accelerator I can hear and feel a bump when transmission is downshifting

Just purchased this SUV.
My concern is: When driving very slowly less than twenty miles per hour, I let off the accelerator very slowly, I can hear and feel a bump that is shifting to a lower gear. I press on the accelerator I can hear and feel a bump shifting back to its higher or previous gear.
Why is this?
With the cruse on and I am going sixty five to seventy miles per hour going down a very steep incline, it does the same thing (bump back and forth to maintain the same speed).
Why is this?
Thanks for your advance reply.
Bob