There's a humming noise coming from my mom's 2011 Honda CR-V LX 2WD 4 cylinder. About 50k miles, mostly city. You only hear it at low speeds. A friend of ours said to change the spark plugs, and indeed it was time to change them, but that wasn't what was causing the problem because we still hear it. There is no check engine light or anything. What could it be? By the way, side question: what's your cat's name and how old is he or she?