Overall im v.pleased with the vehicle but it does have one minor niggle that has baffled me

When the vehicle is cold & has been parked up I get a low speed rumble coming through the transmission, it sounds like a wheel bearing whir but the after a mile or so the rumble goes away & I can just feel a slight vibration through the transmission. Ive had the vehicle raised & running in drive but can't seem to pinpoint the problem.
Ive read many threads and think the problem could lie in a wheel bearing or maybe the front diff! Before I start dismantling the vehicle has anyone else had a simalar problem?
Thanks Carl.