Unknown Sound. Go Kart like?

I know this sounds crazy! It doesn't do it all the time and more often when less than half a tank of gas than when on a full tank. When you press the gas it sounds like a go kart almost. It's not a chirp, it's not a whine, it's almost like ... Go kart explains it best but maybe like a piece of cardboard in a fan but not exactly.. I know this sounds very vaque but we have no idea what this is or what causes it. Any ideas?

(It's not a very loud sound but can be heard easily.)