My car has very loud motorboat noise when cranked. It has been maintained well with regular oil changes. I saw a spring-like coil on the ground under the engine, but I'm not sure it came from my car.
On many vehicles, the exhaust joints are held together with bolts that have a spring coil on them and allow for a little movement as the engine twists under load. There is a possibility this coil fell as the result of a broken bolt and the motor boat noise can be a large exhaust leak. I would recommend having an experienced technician look over the exhaust system to detect the cause of the noise.
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