"I came out, started the car. It sounded like a bomb," said victim Karen Martin.
Her 2019 Mitsubishi Outlander Sport was carved clean of its converter while it was parked on a residential street in Millbourne, Delaware County, just outside Upper Darby.
Automotive experts say catalytic converters, which in short are emission control devices, are coveted by criminals because of their precious metals.
"Inside of (the) casing is your three metals that are very expensive," said Eddie Herskovitz of Main Line Auto Center.
Replacing them after being broken or stolen can also set you back considerably.
SEE ALSO: Mobile vaccination unit hitting Philadelphia streets
"Between $1,000 and $2,000 depending on the car," said Dominic Guaglione of Rosemont Auto.
Reputable auto body shops are expected to return old or damaged ones to manufacturers or face paying a steep penalty.
Upper Darby residents are now keeping an eye out.
"I just hope they catch whoever done it," said one driver.
"I was never aware of it. I didn't even know that existed," said another driver.
Police say part of the problem is people who either see suspicious behavior around a car, or even hear the sound of grinding metal, fail to speak up.
Automotive experts say there are cages you can put under your car to protect your catalytic converter, but even then, there are no guarantees.