My wife's 2004 Tahoe has developed a ticking noise in the engine that only occurs between 1500 and 2000 rpms. I put it in neutral with the emergency brake on and ran it up to that range and it still occurs so I popped the hood, had her run up the rpms and it seems to be coming from the top of the engine on the drivers side. It is a metallic ticking noise almost like an old sewing machine. Any ideas as to what this is? The engine has 104k miles on it.