Purring sound coming from engin

Hi I have a Ford Fiesta style 1.4 diesel on a 58 plate and starting to here a purring sound coming from what sounds like the engine . This only happens when accelerating and going to slow down the sound doesn’t happen when the care is idle?

The car has 135k on the clock and is due to go in for timing belt and water pump doing could the sound be coming from this ?
The car still runs fine with no loss of power

The only thing I’ve had done recent is a A/C regas could this be something to do with that ?

Many thanks
Craig