Couple of simple things to check which cant hurt anything. Pull on the rods underneath the bed and see there is no excessive play---sometimes the cone bearing get out of adjustment with lots of use. Wouldn't hurt to remove the gear covers on the bottom and grease the bevel gears if you havn't already done so. Lastly pry up on the oval cover at spool pin and give a good shot of oil. All basic maintenance.
Knocks are difficult to isolate and I find a careful and sometimes excessive oiling usually solves the problem.
Good Luck
Jon