Maybe he has not checked the oil in 10,000 miles and it's slightly low.
Maybe he has a kid that poked a hole in the muffler.
Maybe the engine mounts are loose.
Maybe the hood is sprung open.
Maybe some under hood sound deadener has fallen off.
Maybe an exhause manifold has cracked.
Maybe he has oil dilution with fuel or coolant.
Maybe he'll be victimized by the first shop that tells him a tale and
charges him $1500.
My guess is it will be the latter.