At random intervals, my 2003 Honda Pilot will make a faint rumbling/shuddering noise. The car itself does not shudder or change handling, it is just a noise, and only lasts a second or two. The rpm indicator dips slightly when this noise is heard, and it goes away as soon as I take my foot off the gas. Usually does not return. Happens between 25 and 40 mph. Sporadic... happens less often in cold weather and after car is serviced (oil change, fluid and tire pressure checks). Mechanic has not been able to duplicate.