Vibrating Noise In Third At 2100Rpm

[redlew]

The RAV, 2005 mark2 diesel manual, has started to make a vibrating noise in third at about 2100 rpm (probably 2050-2150 rpm ) . It doesn't happen in any other gear and it appears regardless of whether accelerating, decelerating or being at a constant speed. The noise appears to come from under the bonnet. Noises are always hard to describe but it is medium to low frequency -almost as if soemthing is just catching.

The DMF was changed less than a year ago and the fact that it only (so far) appears in third at a small rev range hopefully rules that out.

If a gearbox fault, what sort of fault could it be, please?

Thanks for any suggestions -at the moment it is just irritating.