Car revs a little for some unknown reason?

Okay so I've been noticing this for a while. It's a 2003 Ford Mustang Base Couple V6 3.8L and I was sitting in the car waiting for my mom. I was looking at the dial thingy that says RPMx1000 and the dial moves a little even though I'm not touching the pedal and the car is idling. It goes a tiny bit over 1 and makes a noise under the hood for a couple of seconds then stops and goes a little below 1. Also, I don't think this is the case but when I started my car yesterday, I was sitting there for a minute or so and I heard a BOOM under the hood of the car from inside the car and I felt it. It sounded like a person was under the hood and punched it really hard or something. Not sure if this relates to the issue. Anyways, does anyone know what this is or is it normal?

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