I believe you and I have the same boat, a Dufour 31. I cannot remember your year, but it was close to my 77. Mine still has the original Volvo. I also have the vibration you describe. I am not sure if it is as bad. The reason I am so aware of the vibration is it makes the fold down table in the salon area rattle like crazy until the boat is in either forward or reverse. I was told it was because the cutlass bearing were worn and work was needed. Within the next week the boat is being hauled out and this is one of the items on the list. I will let you know if it makes a difference.