(another) strange noise !

hi guys , bit of a weird one here..had the 1.9 octy 4 months now and in the past have done a few longish runs in it (40 miles or so) came back from down south(280 miles at straightforward motorway and a road speeds follwed by a steady drive through my town to the outskirts and parked up and went in the house , came out 10 mins later to move the car , started it up and heard a distinctive reciprical undulating surging whooshing noise not loud but clearly audible , best description is the same sound as something brushing against something as its revolving but its only on idle out of gear its not the engine from what i can hear as i opened the door and you cant hear it outside the car ( iwas sure a street sweeper or machine was nearby so i got out and looked!) ,im sure its not the cooling fans or the heater fan , its only present when the car is thouroughly warmed up after a longer run its most noticeable with all the windows shut ,sound isnt related to engine rpm and it barely audible as the revs increase it does change pitch slightly its not there if i drive off (at least not audible) if i didnt know but was guessing i would say its may be eminating from the gearbox ?? been on local runs and its definately not there , but appears on a longer jaunt, sorry to throw in lots of descriptions but never heard this kind of sound coming from a car before! any ideas?