running westfield with type9 box i know the diff is noisy and needs new crown and pinion but it seams loader in 4th gear is this normal?

Noisier at the same speed? Gear shouldn't make a difference to diff noise; Only speed or load.

^^^ Broadly this.

But I suppose some sort of weird harmonic could be going on between the diff and the gearbox in 4th gear.

The gearbox will be quieter in 4th, perhaps thats why the diff sounds noisier.

With a rigidly mounted diff it could well be a gearbox issue more noticeable or allied to 4th gear, transmitting down prop and resonating at the diff. A spaceframe chassis does make a great sound box and the tiniest rattles can ring like a bell, but not necessarily where you look. I'd check engine mounting and gearbox mounting are tight and in good order too

Has it got more noisy over time?

When I built an MK Indy the Sierra diff was rigidly mounted just behind my bum. I could hear it, but then in a Sierra is mounted in a rubber mounted sub-frame beneath the car so it's going to be quieter.