Rattle noise when I accelerate. Sounds like plastic hitting a fan?

I have a 2003 Ford Focus. Just recently every time I accelerate I hear this rattle noise. The best way to describe it is when you take a piece of plastic and put it to a fan. Or take a baseball card to a bike tire. My check engine light came on and when I had that checked they said it's either the gas cap or I have a very small leak. Could these be connected? Car also makes that sound when I first turn it on.

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