Vectra C Design 1.8 Petrol 08 plate - Weird 'Explosive' noise inside car - Terry2011

I am getting a spasmodic strange noise from within my 08 registered
1.8 Design. It is a noise that can only be described as a sudden load
'wooomph' which sound like all the vents have suddenly closed and
changed the air pressure in the car. There is no effect on the driving
of the car and no obvious event that causes it. It has happened twice
in the last couple of months. It happens at speed 50-70 mph. It is
most disconcerting when it happens. No warning lights no loss of
power. nothing. b***** frightening. Main Dealer not heard of it and
has no idea. Anybody experienced it or has any idea what it is?

Vectra C Design 1.8 Petrol 08 plate - Weird 'Explosive' noise inside car - dadbif
Is it an Irish import?