Vibrating/rattle in 5th

[Lyons]

Not sure if this is an engine or transmission issue, but I assume engine.

Basically when my car (3.0TDi) is in 5th and I accelerate slightly from 35mph there is what sounds like a rattle or vibration in the engine bay area. It stops when i reach 40mph. I don't believe it's labouring to change gear as I would expect it to change immediately down to 4th if it were.

Anyone have any ideas what it could be?!