09 expedtion A/C noise

[hsw55]

First time using A/C since I owned the vehicle. Twice now we have had the AC running when going up a hill, or going from a stop to turning up a hill, and the engine seems to drawing more power, the AC makes a quick sucking sound like a ball being sucked against a open port and once the ball is sucked against the port to close it the sound stops. It only takes a second or 2 for it to appear then stop, and once over the hill you never hear anything. The ac stays running the whole time. Any ideas