I have a 2019 JD 3039R tractor with the Hydrostatic Transmission. When at low idle it makes a ROOOOOT sound. It has been at the JD dealer for two weeks. They are working with the factory to troubleshoot. The shop has isolated the sound to the transmission. When they disable the transmission the sound stops. I spent an hour on the lot listening to two other 3039R tractors and they did not make the noise. Today I was notified that the JD factory said that is a normal sound. I am supposed to pick up the tractor tomorrow. They said it has something to do with air getting mixed with the hydraulic fluid at higher speeds, then when you go back to idle the air collects in larger bubbles and makes that sound when it moves thru the pips. They say all tractors with the Hydrostatic Transmission will make this sound. I have listened to two and used 3 for a total of 5 and have not heard this on any other tractor but mine. What do you think it could be?