DIAGNOSIS That's the sound of the scanner carriage struggling to move. Something is physically obstructing it, usually a bit of paper, a stuck original, or the carriage has come off its track.
FIX Turn off and unplug the printer. Open the scanner lid and the document feeder (if you have one). Look along the glass and the white bar that runs underneath the lid—that's the scanner carriage path. Remove any paper, paper clips, or debris. Manually try to slide the scanner carriage (it's under the glass, you'll see it as a black strip with a lens) from side to side very gently. It should move smoothly.
IF THAT DIDN'T WORK If the carriage is completely seized, there might be a broken belt or a motor issue. This isn't a user-serviceable part. The next step is contacting Brother support, as the scanner unit is a single assembly.
HEADS UP Never force the scanner carriage. If it's hitting a hard stop, you're likely to strip the plastic gears or break the belt. If manual movement feels gritty or catches, it's a hardware failure.