So paranoia has caused me a lot of work the last few days...
I've stripped the front hubs and checked bearings, brakes and steering.
Rear wheels off (not the simplest task...) drums apart and checked.
Exhausts off and split down, joints and silencers checked.
Prop, motor & trans mounts checked.
Clutch cover off and clutch inspected.
Oh, and EVERY nut and bolt on the whole damn car!!!
What I found-
A few rockers needed adjusting.
A front wheel bearing needed tightening (taper rollers).
Drivers door mirror hanging off by one screw! That gave a fright when it csme loose at speed!
Clutch pushrod fouling starter- packed it with washers, now fine.
Drums dragging slightly- backed adjusters off a bit.
Exhaust gasket damaged in first flange joint, right under my legs. This is the only thing I found that could possibly explain the noise.
I took it for a test on Monday and the noise is gone
Took it out this evening and caught the door handle badly on my wall

The door is dented and I can't open the door with the external button. Bananarama!!
Repair begins tomorrow...
No-one will believe you...