Hello,
I wanted to share this with everyone if they ever find out they have a tan mud/sludge in their coolant tank and don't know why. I've heard all sorts of possible suggestions, and when I brought my 2011 Aveo5 LS in, they found out that it was my "engine oil cooler". It's a big metal piece under the exhaust manifold, and there was a leak internally in it which led to oil and coolant to mix inside it and make a sludge in the coolant reservoir. Once they replaced it, it was solved. They drained and flushed out the oil and coolant tanks and refilled both to start from scratch. For repair, expect somewhere between $500-$600, which hurt but it was awesome to finally know what it was. Now this might only be ONE potential fix, but if it happens to you, it's definitely a good thing to check out as a possibility.