I have white smoke that comes from my tail pipes after it sits at idle for a little while(ex. like at a drive-thru or red light). There isn't any visible oil leaking from anywhere that either I nor two other mechanics know of. Could it be the oil separator valve, pcv valve, head gaskets, etc? Is it possibly just water? Usually I can give it a couple of revs and it'll go away. I also can hear a slight rattle/knocking sound. But I only have heard this once and that was today. Thanks for you help and please share your experiences.