The brief
The 325i had a window playing up: slow and jerky going up and down, a grinding or clicking from inside the door, and the glass slipping or sitting at a tilt instead of holding its line. He brought it in. That's a failed window regulator. The regulator is the mechanism inside the door that carries the glass and moves it up and down, on these it's a cable-and-pulley setup driven by a motor. The cables fray, the plastic guide pieces crack, and once that happens the glass loses its support, so it moves unevenly, grinds on the broken bits, slips, and tilts. A regulator that's started to come apart only gets worse, until the window won't move at all or drops into the door, so it needs replacing.
The diagnosis
The door card came off and the regulator told the story, frayed cable and cracked plastic guides, the glass no longer properly carried, which is exactly what makes it jerky, grindy and tilted. The motor itself was checked, and on this one it was sound, so the regulator assembly was the fault. That's a regulator replacement, you don't splice a frayed window cable, so the call was a complete assembly, with the glass realigned in its run.
The work
The door card was removed, the failed regulator unbolted and taken out, and a new genuine BMW-spec regulator assembly fitted, the motor transferred over, the glass clamped into the new carrier and aligned so it runs square in the channel. The window was cycled up and down to set the soft-close stops, and the door card refitted. A check confirmed the window ran smooth and quiet through its full travel and held its line top and bottom.
The outcome
A window that runs smoothly and quietly, no grinding, no jerk, sitting square and sealing properly. The 325i went home with the window sorted. A failing regulator only frays further until the glass drops, so changing the assembly and realigning the glass put it right before it stranded a window down, the kind of job that's much better done before it fully lets go.