Liegh, from a non-mechanical angle, i had this same thing happen slowly with my Rover. Got the loud rumbly noise whilst idling and cruising but not accelerating.

Couldn't for the life of me work it out.

Finally realised the bolt of the bonnet spring at the front had loosened and the bonnet was bouncing around...

All fixed now.

just wondering if it is fuel injected?? coz if it is it very well could be fuel injector/s or it could be the torque converter or tappet clearances??
thats my diagnostics guess/ 5 cents worth

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http://www.youtube.com/watch?v=nDMkVq1jRGU

Yeah it's a fuel injected model. I have it booked into Link Thursday. I'll let everyone know what it was.

Thought I might have sold the White Rover Mini today, but a no show. ASnother tyre kicker but instead of kickin me tyres he just watsed my time, ah well.

See my posting of tyre kickers under general discussion

Thanks again.

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1979 Leyland Moke (which is for sale)