Please don't shout because its a T4

Sometimes when approaching lights/roundabout and slowing to a stop the engine starts to make a really loud tapping noise.
If I rev the engine the noise goes away and may or may not come back later depending if the van can be arsed to be consistent enough!
Alot of people I have spoken to go straight to the common diesel engine fault with brake servo's and if you pump the brake pedal the noise goes away. That does not happen when I try.
Can anybody help with some advice? things to check etc?
Thank you in advance.