The matter is likely to be considered a new threat to aircraft safety.
"It is totally unacceptable," Boeing said in a letter to its employees. Solid objects include: iron equipment, metal carpentry residues, and a rag left by workers behind.
These blocks were found inside fuel tanks on a number of aircraft intended for delivery at a Washington and Texas facility. But the report did not refer to the number of aircraft, saying only that it was during "routine inspections."
The company was awaiting regulatory approval within the United States and abroad to return Boeing "Max 737" to air.
But a company spokesman tried to play down the matter, saying: These blocks do not delay the date of the plane's return to service. Boeing and other international airlines have banned the takeoff of the Max 737, after two crashes in which hundreds were killed.
After the two incidents, several faults were discovered in the plane, such as the problem of the control system, until it became known as the "notorious plane".