Problem is now fixed. The issue, as I analysed was, bad coolant concentration along with minor corrosion in the cooling system. The system was flushed multiple times until clear water was seen. New coolant added keeping 40% concentration of the coolant. 1.6 liter of coolant added and rest water. So, 4 liter system contains 1.6 liter coolant =40%. One of the property of the coolant was mentioned as 'anit-foaming'. As there was lot of foaming, it means anti foaming chemicals were 'worn out'. Further, less concentration of the coolant means water would boil forming bubbles. The orange- brown colour was due to iron corrosion. Thanks all for your comments. There is no foam and no bubbles in the system any more.