I have brown sum what oil looking balls/bubbles(no air in them) on top of my coolant. There is no coolant in the oil and I am not loosing any oil. In the morning when it is cooler there is some white smoke but only for a minute or two during the day it runs clean(no smoke). I don't have a noticeable loss of power. I do have a coolant leak I have to ad coolant once or twice a week. Any Ideas on the brown stuff? Could it be rust or corrosion? Also I recently replaced the thermostat and cleaned the radiator out as best I could. The sludge is only on the top and in the radiator and the overflow the rest is clean.