Three common leak locations on 545i:
- Temperature sensor o-ring on the lower radiator hose
- Coolant expansion tank
- Vent tube (runs under shroud at front of engine bay from top of radiator on drivers side to expansion tank)
Last two will sometimes only leak after engine is shut off (both are involved in "relief valve" role of the expansion tank in the sealed coolant system).
If you have not performed preventive maintenance on your coolant system before, now is the time for a 2005. Belt driven water pumps are cheap for 545i and you'll want to do the thermostat, expansion tank, hoses, belts and fresh coolant. If your vent tube is not leaking now it probably will by the time you get done handling it during disassembly and reassembly. You can either replace the unit (around $40) or replace the plastic tube section with 5/16" copper tube for a permanent fix ($5).
Water pump replacement is not a bad job on the 545i. Biggest PITA is removing the fan and installation of the AC belt (if you don't have the right tool). Handle the plastic parts you are not replacing gently as they become brittle with age.
Consider any other preventive maintenance you may want to do while the car is apart (plugs, air filter, alternator, belt tensioner, etc).
You will have to remove the crankshaft harmonic balancing pulley to get to one of the water pump bolts. Plan on replacing those bolts which are aluminum "stretch" bolts and handle the balancing pulley carefully and set it in a safe place while off the car as they are pretty pricey and can be damaged if dropped on concrete.