Peugeot 3008 gushing under acceleration

Hi I bought a Peugeot 3008 6 weeks ago and so far it's been brilliant.
Two days ago under acceleration can hear a whooshing/gushing noise, the noise is only present under acceleration. The car still seems to be driving fine and no smoke or anything coming out the exhaust. It's booked in under warranty (3 month full warranty when purchased), but they have already said well is there any lights on the dash, there isn't and they have said well it will be hard for us to find the fault then, which seems like a cop out already. I was debating calling the rac out but as its already booked in I see no point. So any ideas as my local garage can look at it but again any signs it's been looked at apart from the dealership voids warranty.