The x-bearings on my Anet A8 had way too much play and were quite noisy, so I decided to try some self-lubricating brass bushings instead (http://s.click.aliexpress.com/e/n6mQbMZ ). The end result is dead quiet and much lighter than the original ones. There are no more bearing noise patterns on the printed objects.
The bushings I used have an outer diameter of 11mm and they can be press-fitted. If necessary, use a long M6 bolt and nut with two large washers and force the bushing into the block by slowly tightening the nut, but remember to protect the inside of the bushing from getting scratched by covering the bolt with tape or paper.
This file already has M4 mounting threads but depending on your print quality, you might want to clean them up with an M4 thread tap.
Note: Proper alignment is very critical with these bushings, otherwise they might get jammed very easily. My solution was to slightly enlarge the M4 screw openings (the ones used to mount the linear bearings) on the extruder bracket and y-carriage to 5mm and to first leave the mounting screws slightly loose, only tightening them (not too much!) after having checked that everything is moving freely. That way, the bushings will automatically find their ideal aligment.
I recommend to run your first few prints at reduced speed to allow the bushings to properly break-in.
EDIT 2.8.2017: Added smaller 10.0-10.5mm ID versions on request.
EDIT 23.7.2017: Added sizes 10.6-11.4mm ID due to requests for further sizes.