Any issues with gearbox mounts ?
When reversing there is a metal on metal vibration/noise around the passenger footwell when under load (like you'd get if the exhaust was installed too close to the body)mounts appear ok but the "dogbone" mount is located below where the sound appears to be when inside the car.
Blipping the throttle can make the noise for a spit second (as the engine twists and returns on its mounting?),but no noise when driving or idling.

Is it a Twinair? I had an issue with the dogbone causing vibration.

Sorry should have said..

7 year old 1.2 Pop, manual, 15000 miles (that's 15k not 150k)
If not a known issue then I'll look elsewhere but mount would make sense with it being noticeable when engine/gearbox twists opposite way to normal (ie reversing).

Exhaust?

My 1.3 does it a little in reverse in guessing mine will be a past its best
Looks a pain to change as the exhaust is in the way of the bolt being removed fully so like need the exhaust taking off
Not the easiest thing to do on the floor