Q: Knock or rattle

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When I give it the gas it rattles or knocks. Only then . I thought it was something rattling, but wound the window down and could hear it outside.

My car has 999999 miles.
My car has an automatic transmission.

The most likely cause of the issue you are having would be a loose heat sheild on your exhaust system. When the gas pedal is being depressed, the engine rpm goes up which increases the flow through the exhaust. This will cause heat sheilds that are loose to rattle. Bumps on the road while driving can also cause this issue to happen. I would recommend having a qualified technician look under your vehicle to check the condition of the exhaust system. If any heat shields are loose, have them reattatched by welding or using some flexible metal wire that can be wrapped around the heat shield. Removing the heat sheild all together is also an option depending on what the heat shield is used to shield heat from.

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