Sounds like a wooden spoon hitting a pot staccatto, ie in time with the engine.

I suspected perhaps an exhaust gasket leak or the dreaded cracked exhaust manifold.

While inspecting the car for oil leaks after an oil change (see other thread), I noted that the sound is loudest coming from the back of the transmission housing, but not the exhaust back box.

What could it be?

Tried sitting on the back of the car?

Maybe the body-chassis bolts are loose?

Also check how welll fastened the floor is too...

Only thing I could think of was if one of the baffles in the exhaust silencer had broken loose.The noise would be in time with firing pulses from engine and sound could travel to rear of gearbox via exhaust mounting bracket.

thanks - that's what I'll choose to believe it is, until I can investigate further...

I replaced the flat exhaust mounting plate between the backbox and the transmission. The old piece had a crack which had been welded and re broken along the weld. The replacement piece sourced from SJ is OEM Lotus but of a thicker gauge steel, and with additional flange on the bottom, so this must have been a common failure.

Anyway, everything came apart and went back together without any difficulties and the car is now purring as it should.