Engine grumble at low rpm

[Lockyspain1975]

Hi Guys 

My 3.0tdi 2012 has developed a strange grumble sound. It doesn’t happen in neutral only under load. It seems to only happen at around 1500-2000rpm and to be honest just sounds like a regular diesel clatter sound but these engines are so refined it’s just odd. The strange thing is it’s intermittent sometimes no noise at all and the car seems smoother or it could just be me. I have already had all chains and tensioners changed along with 6 new injectors. I’m thinking maybe Swirl Flaps? Not opening properly and making this noise. Any help would be great 

 thanks