I have a 2008 VDP. Have humming noise at 50-55 mph. Accelerate and it used to go away. Now lasting longer. Also getting a rumbling when accelerating moderately after slowing down to 30. Don't know if they are related. Transmission shifts fine and car runs great. 115,000 miles on the car.

Any idea of what this could be?

 

First let me welcome you to the forums. One of the moderators will come along soon and direct you to the new member section for a mandatory introduction. In the mean time your vehicle is an X350 designation and this is the X308 section. You should repost in the X350 section of the forum and you will get a better response, since the members there have a like vehicle.