The warning sirens sounded in Pickering, Ont., on Tuesday, a test of the nuclear facility's emergency system.
But for most residents it was just an irritating noise that left them confused.
"It sounded like bells, or a train, or something like that," said Rahim Mohammad. "I didn't know what it was."
Ivan Ciuciura, the emergency services director for Durham region, said the province has mandated the Pickering plant have a warning system that warns residents of an accident immediately.
"It's attention getting signal, so if you hear it it just means that something may have occurred at the Pickering nuclear plant," said Ciuciura.
Tuesday's field test was to check to make sure the sirens could be heard within a three-kilometre radius of the plant.
"We know they work," said Ciuciura, "but this is a final test to make sure we have enough sirens."
If they're not they'll have to increase the number of sirens in the region.
It will be two or three weeks before testers know if the sirens are loud enough.