heater issue

when I have the heater on in my 2000 zx2 you can feel the car jerk ever so slightly what is causing this issue any answer will bring me that much closer :shakehead

A/C could be coming on at the same time. this would be normal for defrost to dry out the air. turn on the heater and look at the ac coompressor to see if it is running. if it is, then that would more than likely be the cause of the light jerk.