A couple of months ago I bought a 15,000 2008 BMW 330i SE Auto (E90) from my local main dealer. Upon collection, I noticed a number of noises from the engine area which were cause for concern. In addition, the coolant level was well below the minimum mark. I advised the dealer and returned the car to them for investigation at the first available opportunity.

To date, the car had been returned four times and had various work done, including a fuel injector and ignition coil replaced on number one cylinder (twice), all spark plugs replaced, a new NOx sensor and software update, all under warranty. Most of the noises have been cured. However one, a regular soft knocking sound, remains. The problem can best be heard from inside the cabin at idle and is more noticeable when Drive or Reverse is selected while stationary. It occurs most of the time but not always.

I understand that the lean burn process means the engine tickover is generally noisy on these engines but the problem I describe here is distinctly separate from that.

The dealer has been most co-operative at all stages and agrees there is an unusual sound. However, extensive investigation, including use of a stethoscope on engine and auto box has revealed nothing. They have suggested we wait until a similar car comes in for sale or repair and we can compare the two to see if this is normal. However, these models are not that common and so this may take sometime.

The engine also runs a little unevenly when cold, with mild hesitation under light throttle. Once warm however, there is a lot of smooth power.

Over 1000 miles or so the car has continued to lose coolant. The level has fallen from the max to the min mark over that distance. Extensive examination and hot and cold pressure testing by the dealer, including overnight, has revealed no leaks whatsoever.

Can you suggest anything that may help please? The car is otherwise in superb condition and these apparent faults have marred ownership so far.