It comes as the 737 Max remains grounded after two fatal crashes.
The US plane maker said it discovered so-called “Foreign Object Debris” left inside the wing fuel tanks of several undelivered 737 Maxs.
Foreign Object Debris is an industrial term for rags, tools, metal shavings and other materials left behind by workers during the assembly process.
The aircraft has been grounded by regulators around the world since March 2019.
It was banned from flying after two separate crashes killed 346 people.