advice needed on strange engine noise

I'm thinking of buying a UK spec Focus 2.0 Ghia (5 door, petrol, manual, 2001 reg). I took it for a test drive today. Starting engine from cold,
all sounded well enough and the car felt solid on the road. However there was a strange engine noise below about 2500rpm that has me a little worried. I can best describe it as like demented pinking (but not pinking)! It's quite noticeable and happens in any gear, but obviously you hear it more in 3rd/4th as you can keep the revs there (needs some throttle to make it happen - dies down at constant revs).
The car has been off the road for almost 6 months, so probably has yucky deposits at the bottom of both the petrol and the oil. They're offering to do a mini-service and MOT for free. The service would include changing the oil and filter, flushing the oil out and the fuel system with a cleaning solution. They say they'll remedy any faults (it will have to pass the MOT anyway) and give a 3 month warranty on major problems.
Unfortunately I only got to speak to their sales guy and not one of their mechanics and he didn't know what the noise was. Do any of you have any idea at all what it might be? The car has some nice extras in it that you don't often see and is in good condition otherwise so it would be a little bit of a shame to let it go, but not the end of the world!
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I found this when browsing around and it sounds a lot like what I experienced:
http://forums.focaljet.com/zetec-tuning/420335-low-rpm-2000-engine-noise.html
Has anyone else come across that?
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