The most likely reason - the steering rack has a leak and it is time to repair or change it. Oil gradually flowed out from the power-steering system and a pump began to howl. At the same time it will be harder to turn the steering wheel. And this is trueth not only for bmw 3 series, but for all cars.
Typically, refilling liquid in the tank you will solve a problem for a while, but it is necessary to eliminate the flow because further will be only worse.
Pump howling should not be confused with a squeak of bearings steering shaft. The last ones exactly coincide in time with the steering wheel movement unlike the pump, which may be slightly delayed after the wheel movement. And as a whole, it is not difficult to distinguish the howling from the squeak.