The car exhaust system contains a silencer (also known as a muffler) its job is to minimise the noise that comes from a car’s engine. Silencers contain perforated tubes or baffled chambers that create opposing sound waves when sound waves from the engine travel into the silencer. Not only that but if the car’s silencer has a hole in it carbon monoxide can leak into the vehicle.
Informally, this is a business that can be run from the side of a road with a few ramps and a gas welder, but if you can get space in a garage it will be even better (depending on the area, you may not be allowed to offer your service on the side of the road).
Image credit: brokkenstar