The engine is starting to make a knocking noise.
A knocking noise is never a good sign on an engine. If the sound appears once, then the engine is started and gets louder when you rev the engine then it may be a rod knock meaning that the engine needs to be rebuilt. It can also be caused by an issue with one of the engine accessories or the timing belt. To isolate it, I usually remove the belt and start the engine to see if anything changes. The only way to know for sure would be to listen to where the noise may be coming from on the engine. If you wanted to have this looked at by a professional, consider YourMechanic, as a certified mechanic can come to your home or office to diagnose the noise and recommend a repair.
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