A beeping credit-card scanner last night set off a bomb scare aboard an airliner preparing for takeoff at La Guardia Airport, authorities said.

Ninety-nine passengers were evacuated from the 6 p.m. Boston-bound US Airways Flight 2138 after a suspicious sound was heard in the cabin area, according to an airline spokesman.

An inspection revealed the culprit to be the low-battery indicator on the portable device, a Port Authority spokeswoman said.

The flight departed without further incident.