Boeing called off its first astronaut launch because of a valve problem on the rocket Monday night, Associated Press (AP) reported.

The two NASA test pilots had just strapped into Boeing’s Starliner capsule for a flight to the International Space Station when the countdown was halted, just two hours before the planned liftoff.

United Launch Alliance CEO Tory Bruno said an oxygen pressure-relief valve on the upper stage of the company’s Atlas rocket started fluttering open and close, creating a loud buzz.

The valve may have exceeded its 200,000 lifetime cycles, Bruno said, which means it would have to be replaced, pushing the launch into next week. But if engineers can quickly determine that the valve is still within that limit, the launch team could try again as soon as Tuesday night.

It was the latest delay for Boeing’s first crew flight, on hold for years because of capsule trouble.