The front left of the vehicle full stop, it's coolant guaranteed 100%.

Mr BMW thanks will look for that.

Everything on the front of the engine (Water pump and surrounding pipes, Radiator hoses) look fine and it seems to be an intermittent thing. Sometimes it leaks bad and the expansion tank empties itself completely and sometimes I don't get any leak. I've had a look at some of the coolant system schematics and I can't seem to find anything in particular that it could be but is there an overpressure valve somewhere that may be dicky?

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2004 E53 4.4L N62