N52 engine on your '06 car has electric water pump. Water pump is a known weak link on that engine's cooling system. I just bought a Pierburg electric water pump for my '06 530i (with N52B30 engine) on Amazon for $345. That's the lowest price I've found. I am planning to replace the water pump as a preventive maintenance - once the weather gets little warmer.

With mechanical water pumps, you get the tell-tale signs as the pump goes out - like the coolant leaking from the weeping hole or the pulley wobbles or making noise. Electric pump seem to just go out without much advance notice.

 

My 2004 545i SMG has about 106,000 miles now.

About every 3-4 weeks, I am getting a low coolant warning and having to top off the coolant. Is it possible that I might need to consider this water pump replacement? Would that be a possible cause for this loss of coolant?

 

2 possibilities -
Coolant level sensor in the coolant tank is malfunctioning and giving erroneous results. Since you are actually topping off the coolant. So, I am assuming this is not the case.
Next possibility is coolant is leaking from one of the hoses/connections at the radiator or water pump or thermostat. Only way to find the source of the leak is to use a flash light and follow the coolant lines and see if there is a coolant (or dried coolant residue) leak. Check the connection of the thin hose that connects from coolant tank to the top of radiator. It is known to leak. Also, check the coolant temp sensor on the lower radiator hose. It's known to leak as well.

Without finding the actual root cause of the leak, replacing water pump is not a logical thing to do.

 

Third possibility, and worst, is head gasket or water jacket is busted. Do you have any water or foaming/froth in your oil after running?

Open your oil filler and smell - smell like coolant?

You are refilling with BMW coolant, right? :-)

good luck,
Mike