Laguna 3 dCi louder than usual

I have a 2008 2.0dci Laguna 3 that has never given any trouble. Two days ago while driving on a back road I noticed while revving up through the lower gears my engine sounded louder than usual. It sounded like a whooshing noise but once up to cruising speed sounds fine and refined again.

When the car is parked and I rev it there is no noise, it is a strange one as there are no warning lights and no performance issues.

Has anyone seen or heard this and know the potential origin?

Laguna 3 dCi louder than usual