Sounds like some oil is leaking in there. If you've changed it repeatedly, it's not the coolant.
...my coolant was brown, then I realized I had a busted head gasket and oil was leaking.
It sucks, but as Mr. Bill would say: "It's just money."
I know, that's easy for me to say since I got mine back this week. It's a little less painful when you've survived.
Good luck finding out what it is buddy. I hope it is NOT a head gasket, but not many things can cause your coolant to be brown; especially after repeated changes.