Follow the steps at the below link to clean the drum unit:
Cleaning The Drum - HL-4040CN
If this does not fix the problem then most likely the drum will need to replaced with a Brother DR110CL drum unit. The drum has an approximate life of 17,000 pages. If you have changed the cartridges a few times then a drum replacement may be due.
Some units require a replacement earlier than others. If the unit is under warranty then service may be required by brother especially if the feel a drum replacement is premature given the approximate service life of the machine.
Locate An Authorized Brother Service Center
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