I know the sound you are talking about. It is just the clutch slipping slightly as it's engaging. It will probably disappear with use. Maybe your dealer never burnished in the clutch for you, which is simply running the machine at WOT and engaging the PTO for a couple seconds, then disengaging, waiting for 30 seconds to a minute for the clutch to cool, then repeating the procedure for ten times. After that, engage the PTO as outline in your ops manual. Yours may say at WOT or at idle. I've had JD's that recommended both, but they normally recommend engaging the PTO at 1/2- 3/4 throttle. I always do it at the lowest speed I can that allows a nice smooth engagement without belt slap, or engine loading down much, if any. For my Z950, that is at approx 1/4 - 1/3 throttle.