I was listening to my car earlier whilst parked up idling, and after kneeling down on the pavement to listen further down under the engine I could hear a very faint intermittent whirring/grinding noise. The noise is quite faint, and only comes every 10 seconds or so and lasts for a second or so, the rest of the time there are no noises and it all sounds sweet. Any ideas what this could be? A drive belt pulley? Tensioner? DMF?