Just had a thought...about a month ago I changed my air filter and noticed a small hole next to the left rear screw in the air box cover. Would this have any affect. Air is being sucked through the air box so maybe it's affecting the idle when the revs drop and he car rumbles. Am planning to remove the whole air box and put a cone filter on anyway. Would a hole in the air box give the symptoms I'm experiencing? I haven't seen a mass air flow sensir on my car so can't check that.