2006 Subaru "Bumps" when changing pressure on acce

Other Category problem
2006 Subaru Front Wheel Drive Automatic 35000 miles
Garage name : Authorized Subaru Garage

I drive a Subaru Impreza, 1600cc car that I recently purchased from a car rental company. When changing the pressure on the accelerator, usually when first decreasing and then increasing the pressure, I feel gentle "bumps". Though it'is probably not the reason, it is as if I can feel the gears changing. My authorized Subaru garage checked it out on computer and found nothing. The phenomenon, nevertheless, continues. What might be the reason for this and how can it be corrected?