Engine knock and smokes

The car runs great. But sometimes, like once or twice a month, if the car is sitting in a drive through or a traffic light, it has a knocking sound coming from under the hood on the driver's side and blows out a huge White cloud of smoke from under the hood and tail pipe. When the car starts off it stops the knocking and smoking. The smoke has a smell of antifreeze but does not use any coolant. It only does it a few times a month. Rest of the time it runs fantastic. I just wonder if it might be a injector or a piston. I don't understand why it runs so well and has these episodes. Any light you can shine on this problem would be greatly appreciated. I love the car but some local mechanics I've asked about it don't know what it could be and their solution is to sell the car.