Loud 'Click' From Bell Housing

[ferret76]

I'm getting a loud clicking noise from the bell housing between 60 - 70km/h. I'm told it is loud enough to be heard outside the vehicle. I have recently renewed the engine and gear box bearings and can't detect any unusual noise coming from either. I recently had the sump off and noted a tiny bit of front to rear movement in the big ends (literally a hair's breadth)I have adjusted the tappet clearances and have discounted the fuel pump as although it makes a noise it is no where near as loud as this sound.
To my ear, it definitely sounds like the something in the clutch is hitting something as it rotates but a recent inspection of the clutch, release bearing and fork shows no indication of this.
Has anyone else had this annoying click? It is quite loud and occurs at the speeds that I drive at all the time, a bit slower or a bit faster and it goes away.
Any advice would be welcome.

[bmcecosse]

And there is NO noise at any other speed??

[Chipper]

Is the gearbox retaining cable in place and correctly tensioned?

Can you feel any drivetrain 'thunk' as the noise occurs?

[MarkyB]

How do you know it's from the bell housing?
Might be worth raising the rear securely on axle stands then running up to speed to find it.