Hum from engine

[310D]

Hello there, my name is Kenn I own a 310D LWB sprinter. Recently there has been a strange hum from the engine of the van when I hit 40mph in 4th gear, if I give it a bit more juice the hum disappears but in 5th gear it will come back at fifty for a short while with the same results, Is there a van doctor here that might know the problem. Help would be much appreciated as in Ireland just to have a mechanic look at it takes alot of time and lost money.
Thanks