What could cause a knocking sound under the hood of a car?

I just bought a used 2013 Ford fiesta last month from a dealership. Yesterday, I did a 3hr trip to visit my gf in west Virginia. I pulled off the freeway, because I felt like I was lost. GPS wasn t helping me at all. So, I m going like 25mph and I notice the knocking sound under the hood. It sounds like it s coming from the driver side of the car. I still visited my gf, but I was cautious all the way home. I only hear it when I first start up the car and at low speeds. When I go above 10mph I can t hear it at all. I got an appointment with a mechanic for Friday, but I work all this week and I m making another trip to west Virginia on Wednesday. Is it safe to drive or is this something serious?

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