The pump should not operate when going from R to D. It should operate when going from P to any other position so it sounds like yours is working the way it should. The most common failure is the leaking of the green pressure switch inside. You can check the fluid to see if it is low and remove the back plate to check inside to see it the pressure switch is leaking. After you talk to Roger let us know how you make out. Roger has helped me several times. The switch is available from a Chevy dealer and is about $50. I have changed this twice and now carry a spare.