White smoking on idle

I have a 2007 A4 avant BRD 170, it’s developed a smoke on idle just the other day, I’ve discovered that if you release the pressure out of the header tank the smoking stops, I’ve changed the oil in it this morning and left the old oil that’s came out in a clear plastic container to see if the oil has any water/coolant in. Good news, it hasn’t.. also checked inside the coolant header tank for oil and it’s clear just coolant in there, when it’s smoking and I hold my hand over the exhausts it doesnt make my hand damp or wet and doesn’t taste sweet, so hopefully that’s eliminated head gasket. I don’t think it’s injectors as the car still pulls hard in every gear and doesnt smoke while under load, it’s as smooth as anything when idling no rough shakes or juddering, I think if the oil cooler was faulty then there would be oil in the coolant as oil is higher pressure than water and would force it’s way through, coolant is clean so can’t be as far as I’m aware.. I have no idea what else it could be other than the tandem pump but I have no idea how they work, anyone have any suggestions? Would be much appreciated