Blowing noise

[Fatmackem40]

Hi , I’m new to this forum.
I have a 1.6tdi 2010, recently I have heard a noise coming from the engine. At first I thought it was the exhaust but, can’t find or hear anything when car is idling or rev’d.
I did notice the noise, which is like a farting noise, happens when accelerating and under load. It is reasonably load, but the strange thing is it doesn’t happen all the time.
Any help appreciate