Morgan 4/4 Petrol Tank

[NIGEL BALDOCK]

My Morgan is approaching a year old. Since new upon opening the petrol cap to refuel, there is the sound of inrushing air (I think - although my supplying garage suggeststs it is the reverse) accompanied by the sound of the tank returning to its original shape. The garage tell me this is quite normal and whilst I believe them (of course!!) does any one else have experience of this???