Normal oil consumption according to Alfa can be upto 1 litre per 600 miles, although most engines don't use any where near that much.
It is advised to check the level weekly or after a long journey.
The rattle could be a loose exhaust heatshield, or a cracked manifold. If it is a cracked manifold it is often worse when the engine is first started and as the heat seals up the crack.
You also say the coolant is brown, it that because it has oil in it? if so you may have a failed headgasket or oil/water heat exchanger.
Giulietta 2.0JTDm (170), BMW 120d M sport