Injectors? Or Gearbox Service due / Low oil

[X6M50d2025]

Hi All

I'm wondering if anyone has experienced this issue and any ideas...

E71 2014 63reg X6 M50d, now on 110k 1 prior owner, FBMWSH upto date.

So.... in January I experienced a noise under accelerating only which sounded a bit like an injector knock or due a service.

Oil level good, never let me down in 10k, no smoke, average 29.5mpg, no power loss, no rough idle, no issues at all runs like new.

So I had a full Service with best recommended oil including all filters and fuel filter. It didn't solve the issue.

What I have now found is when driving under acceleration you get the noise but not if coast / slowing down. Its subtle and misses doesn't notice it, can't hear externally or if drive in a multistory etc just in cabin. 

Today I've found if parked in D on tick over I hear it how ever if in N I don't and not even when rev...

Is it just a high miler issue / injectors tired but OK or something else like low Gearbox oil?

Google states noises under load in Drive and not Neutral can be transmission or Injector so thought I'd ask people with a clue not google. Worst case I'll have Gearbox ZF Service done and injector test on all banks.

Thanks in Advance.