I have being trying to diagnose a coolant leak on my '05 545 for the past month. I don't see any leaks from the water pump or the timing cover area (the dreaded coolant pipe leak) while running. It happens over night or if the car has been parked for 3 or so hours.
I have ordered a new water pump and thermostat and plan to replace them this weekend. The pump is growling now and sounds like the bearings are going.
It "looks" like the leak is in the front area of the engine and sometimes it is pretty substantial from what is on the ground.
Anyone have experience with this or any thoughts/ideas?