2003 BMW 325I Power Steering

What kind of bit are you putting it in? Transmission? Engine? Power Steering?

The power steering reservoir should be just under the oil filter housing (round black case with a large nut on the top). It should have a star-shaped handle on it.

E46 BMWs use ATF in their power steering so make sure you use the right fluid.

and you need to replace it two months later, again.

hose or connection most likely has broken, have you lost fluid aswell? take it back to the garage who did it they might sort it out for free as long as it's not a burst hose

Also note, in these cars they use AUTOMATIC TRANSMISSION FLUID for their power steering system, so don't be alarmed at the colour.

most power steering caps have dipstick attached to the lid,and show a level for warm and cold, if not check the level is approx 1inc below neck...

Check the steering rack to make it's not leaking, also verify the fluid level in the pump.

It might be a faulty power steering pump. Try checking the level of the power steering fluid. It is probably best you have your power steering system checked by a mechanic.

Does the noise occur only when the vehicle is moving?

Try moving the steering wheel from side to side, while stationary (but don't do it too often, or you might damage something). If the noise is still present when the car is stationary, then it probably is the power steering system.

If the noise only occurs while the vehicle is moving, it could be a problem with the differential/final drive, or even wheel bearings.

have the power steering pump changed or fill the power steering fluid

Make sure you check your power steering fluid... if it's even low, it will make a terrible noise...run it on empty too long and your can ruin your power steering pump.