U.S. watchdogs have already grounded 171 of the planes after a panel blew off an Alaska Airlines flight last week.
Pilots were able to land the crippled aircraft without serious injuries.
But now United Airlines says its has found loose parts on some of its own 737 MAX 9s.
The carrier says it found bolts that needed tightening on several panels.
Reuters sources say United has so far found close to 10 aircraft affected by the problem.
Late Monday, Alaskan said it too had found what it called "loose hardware".
The news will raise fresh concerns over how Boeing's best-selling model is manufactured.
National Transportation Safety Board Chair Jennifer Homendy said Monday that the cause of the blowout remained unclear:
"We need to first and foremost, figure out what happened here on this aircraft. If we have a bigger system wide or fleet issue, we will issue an urgent safety recommendation to push for change."
MAX jets were previously grounded for 20 months following two fatal crashes that killed 346 people.
Those accidents were traced to a design flaw that has since been fixed.
Now aviation experts say the new problems raise real questions over quality control at Boeing and supplier Spirit AeroSystems, which makes the panels.
However, the part that blew off is only fitted to specific variants of the MAX.
United and Alaska Airlines are the biggest operators of the affected type.
Other carriers to ground planes include Turkish Airlines and Aeromexico.
