Started hearing a metallic rubbing sound from engine. Like something rubbing against a case cover or ?. Thought it may be the water pump but the noise is coming from the other side. Does not knock when started but after running a few minutes starts low and gets louder as it runs. If I blip the throttle it goes away and then returns after a couple of seconds. With my stethoscope the only place I can hear it is on the left front exhaust pipe. Scraping sound with ticks about every second. Any ideas what this may be.. Would valve shims make such a sound?