Xantia 110HDi Estate - vibrations?

[neilminto]

Car is March 2001 registered Xantia 110 HDi Estate - now 28k miles.
At 25k miles, approaching service, the steering wheel was vibrating very slightly - didn't feel particularly like wheel balance, just irritating 'tingle'. Tyres were down to 3mm anyway, so I had 2 new Michelin OE tyres fitted at 25k service.
Problem didn't go away - if anything marginally worse, plus definite periods of wheel balance shimmy. After several rebalances (last one by a good tyre dealer sudcontracted by the dealer, using new equipment, watched closely by me!), that element is now gone but it still has an annoying 'tingle' through the wheel - worse, but not exclusively, on slightly rough surfaces. Dealer (car still under warranty) 'can't really decide whether its a problem or not' (well, guess it isn't for him except when I'm there!)
He has also checked ride height, sphere pressures (allegedly) etc and thinks if it persists he may start changing spheres.
Any thoughts on this - should the ride have become jiggly and the steering wheel 'tingly' at this sort of age/mileage? If so what needs doing? How long do spheres last? The car does a 50 mile round trip commute daily on reasonable roads and rarely carries weight - just me.
Obviously being one of the very last registered it may have stood for a bit, although the 110 Forte estate was only on sale from Oct 2000 (I think?) so not for too long - could that make a difference to sphere life? Or is it likely to be something else entirely?
Any thoughts please?
(Otherwise - delighted with it!)
Thanks
Neil

[JohnD]

Front spheres should go for around three years before changing. Have the brake pads been changed?

[neilminto]

Thanks John - Yes, the pads were changed, together with the front discs at circa 20k - could that be significant?
Neil