Have you checked your power steering fluid condition lately? It looks like the previous owner of my 269,XXX mile 328i accidentally filled the reservoir with ATF instead of CHF11S (designated by the green sticker on the fill cap)!
I immediately purchased a new genuine reservoir, as the filter is built into the container itself. New factory clamps, a liter of CHF11S and we are on our way.
Also note, this DIY is specific to E90's *without* active steering. That is a different reservoir. Do also take note some cars may specify ATF. Finally, it is very common for the green CHF11S sticker on your fill cap to fall off over time.