My ML 500 just started a new behviour. It now spurts while idling. If you listen carefully, you can hear a random spurting/rumbling or what sounds like a random base drum thumping from time to time in the backgound. It is more obvious when you open the hood and listen around the MAF or behind at the exhaust. I wonder what this is. My mechanic tells me it is one or more of the plugs misfiring and that I need to change plugs. Is this true? I need other opinions. Please help.