Difficult starting and noisy when warm

[Alanholt]

Hi, my van is a 2008 2.2 140.

When the engine is cold, van starts instantly, ticks over nice and smooth and is as quiet as when it was new.

Sometimes, if I boot it, it will say engine malfunction and I lose boost. I can simply knock it out of gear, let the revs drop whilst the van is still rolling, rev it once, put it back into gear and boost has returned. This may happen once every 700miles.

As the van gets warm, a gentle tap starts to come from the top of the engine, but I can't identify where, even using a stethoscope. It sounds as though all the injectors have suddenly become really noisy. When the engine is held at 1400rpm and 1800rpm, the noise is worse, but away from these engine speeds, its just a little louder than normal. 1400rpm and 1800rpm are the speeds it spends most of its life on cruise control, if that matters. Also when its warm, the engine takes about 3 seconds of cranking before it tries to tickover. It cranks at normal speed, but is slow to fire, and when it does, takes a while to get to normal tickover speed.

Let the engine cool, its faultless again.

To anticipate some questions, fuel filter is new and genuine (less than 2 weeks) and full to the top. No fault codes using an ODB2 scanner. Doesn't smoke, doesn't use oil, great on fuel (7.1l/100km or 33mpg). Van still pulls like a train unless the first fault occurs. I know its difficult to diagnose, but someone may have had the same symptoms and be able to say 'mine did that and it was this'.

Thanks in advance

[Chug]

Loose/broken injector bolts/clamps?