That usually means oil or coolant is involved in the combustion somewhere if it doesn't go away when the engine is warm. The usual source is misfiring due to a foiled or malfunctioning spark plug in one or more cylinders. That's best case scenario. The worst case scenario is a head problem or head gasket problem that may be allowing oil and/or coolant into the cylinders. Get it checked by a pro ... Good luck

Hi Pelegrin! Generally if this continues after the engine is warm it's the result of oil and/or coolant in the cylinder. The good-news end of the scale here is that one or more spark plugs is fouled (dirty, failing, cracked) and the mixture in that cylinder isn't being burned properly. That's two bucks per plug and a few minutes in the back yard. The bad-news end of the scale could be  a problem with the head and/or the head gasket which is allowing coolant into the cylinder. You need to have it checked out by a pro. Good luck!  

Also, tell them to check the Intake manifold gaskets.