:( Yup, 'fraid so. From my experience GM'* stupid idea of using the "sealing tablets" (-ie stop leak, so as to "band-aid fix" the real problem) does in fact cause brown gunk in the coolant. -especially noticeable in the coolant recovery bottle.
I was lucky enough to snag my '02 from a dealership, BEFORE the put their stupid tablets in. So at 44k miles, that one was still clean. -BUT, of course, it is starting to leak coolant now under the throttle body. I have the new Upper Intake Manifold in my garage, just waiting for the ambition to strike me. (-won't need to drive it, until I get it fixed)
And BTW, on my '99 which had the brown gunk problem (along with all the other GM design screwup problems) it took a lot of rinses to get most of the brown sediment particles out of the system. Also, I removed the radiator, and took it to a shop here in town that has a large ultra-sonic cleaning tank. WOW, it came out looking like new !! -and for way cheaper than a "radiator shop" cleaning.
Dark Cherry 99 Bonneville SE -her car
Ivory 02 Bonneville SE -MY CAR !
"luxury with attitude"