when i crank up my 1998 f150 it knocks when it is cold but stops when it warms up. does anyone have an idea what it might be.

 

It could be the oil taking time to circulate thru the engine. You may be using oil that is too heavy for the winter. A lighter weight will circulate more easily when it is cold outside.

Just my $.02, I am sure others will have more suggestions.

Good luck!