Agree. The noise should only be noticeable at full lock. Normal pump noise shouldn't be noticeable from within the vehicle. With my limited mechanical experience I would guess a slowly failing pump or maybe even power steering fluid...?
F30 328i Modern Line
E83 X3 2.0d
Retired: '07 X3 3.0d M-Sport - '04 X3 2.5i Sportpack - '99 328i E46 - '93 325i E36