I think it's just Dexcool breaking down and maybe the intake gaskets going bad.

 

thank you for all the advise on the brown junk in my radiator. i just bought a 2004 ss supercharged 3.8l. i noticed the brown mud on the radiator cap and figured it was bars leak or some other sealer. i thought about flushing it and putting fresh anti-freeze in it. it doesn't run hot at all even in traffic the gauge sits at about 1/2 way. its not burning or loosing coolant.
i had the same problem on my 2002 ls 3.4l with coolant leaking into the trans cooler. i ended up just replacing the radiator and changing the trans fluid and filter. it has over 300.000 miles on it and still working no problem.

 

Thankfully I had coolant system flushed and refilled about 6 weeks ago and I don't have anymore black gunk. No bubbling sound under the dash either.