Hi all, ive had my w reg A170cdi for about 6 weeks and it was running fine until 2 days ago when the engine started making a knocking noise on tickover. this has got louder but as soon as the car is revved or driven it sounds fine. ive checked the pulleys and they seem fine. it was serviced approx 4000 miles ago and all fluid levels are fine and no obvious leaks. has anyone got any ideas of what to check next ?