i have about 25k miles on my truck now and have recently noticed heavy grey smoke and a bit of a rough idle when i start up. it only does it sometimes, usually in the morning or in the afternoon after the truck has been sitting and the engine is completely cool. outside temp does not seem to be a factor. it has done it with 70 to 30 degree outside temps. anyone have any ideas as to what this could be. after the initial smoking it clears up within about a minute and no longer smokes and idles fine. Thanks for any input

Stephen