I have a 2015 Forester and when the vehicle is accelerating or I'm going up a hill and give it more gas, sometimes a clackity-clack noise comes from the engine. It's not constant and it doesn't happen at idle. Seems to happen in warm weather more often. The best I can describe the sound is it's like when your bicycle chain is sort of between gears. I guess it could be valves tapping. It happened on my old Forester, but that one has 200K+ miles on it so paid it no mind. Is this normal?