Audi a4 2ltr tdi strange engine noise!! Help.

[Chops77]

Hi all. My first post so please be gentle.

I have just purchased a 2005 audi a4 avant 2ltr tdi with 81k on the clock with fsh and the car is immaculate.
It has had the head and oil pump changed about 5k miles ago due to the common head cracking issue and oil ending up in the coolant,anyway that issue is sorted but thought maybe worth mentioning.
So this noise that is happening only occurs from low revs (1000-2000rpm) when driving and only if u are quite heavy on the throttle. Once past that rev range then the noise dissapears. Its a hard noise to describe but is almost an engine clatter type sound, like if you were about to stall but catch it!
I work at a main vw dealership and have had the mastertech take a test drive in it and have a listen and he cant determine what the noise is but suggests maybe its been badly timed when the head was replaced!!
Just wondered if anyone else has had a similar issue or noise or can shed some light into what it could be.
Many thanks for taking the time to read this and hope to hear back soon.