Engine rpm goes down to ~1000rpm when not accelerating or maintaining speed

What is your question?

New care to me. If I engage Cruise Control at speed e.g.70mph seemed to work fine. If I disengage by touching break and then resume using the cruise control the car returns to speed but the enging speed drops to less than 1000 rpm e.g. if going down hill. The cruise control is still engaged because it will acclerate as needed. I can hear a whinning sound as the engine comes back up to speed.