The BMW Diesel engine should be good for a lot more than 225,000km - I would expect 500,000km from a diesel unit in this day and age. Petrol units don't last that long. In any event it doesn't sound like the power unit itself is faulty, just the ancillaries.

One thought about the clicking - does this only occur when the air temp is also particularly cold (5degC or less)? It might be the air conditioning preheater unit. Mine certainly 'clicks' for the first 5 mins and then stops. The exhaust unit is under the car (left hand side on my right hand drive 2005 X5). The unit is mounted on the right side of the engine bay close to the front suspension upper turret as you look in from the front and made by Webasto. As it's exhaust runs down that side, it might fool you - but then again........

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