Freelander td4 engine snack/knock??

Hello everyone, first post on here, been looking on the forum but can't find this problem anywhere:

I have a 06 Freelander TD4, it runs well but when I rev it up, as it goes through around 1500 rpm there is a single snack noise coming from the engine.
It doesn't do it every time - maybe 1 in four times or so and both revving stationary and when the engine is under load.

Any help would be appreciated.

Thanks
Matt