Very audible in the cabin, but only at 1200 rpm. It sounds like something vibrating against the actual chassis.
I had a brake pipe fracture that runs across the back of the engine (a real trouser browning moment), and the stealer had big problems fitting it. I reckon they dislodged or fudged something then, but can't prove anything.
I've had a look today and there's a very flimsy bit of metal on the bulkhead that looks like a heatshield or something. Have tried pushing it in a bit but I don't think that's the
problem unfortunately.
Ultimately I'll have to see if it can be fixed under warranty. If not I'll have to live with it.