While I'm waiting for my heads to come back from the machine shop (blown head gasket...both it turned out), I'm cleaning things and prepping for the installation. While cleaning out my coolant reservoir, I heard something knocking around in it...I get it out and it was two big hunks of what honestly looked like clay, stinking of antifreeze/oil. Is it possible that somebody put something in the coolant system before I got the car to cover up an already-blown hd? Or could this just be normal accumulation of gunk in a 15 year old coolant system...this car was not modified whatsoever when I bought it so it's possible that the reservoir has never been removed (until now). Just curious more than anything...do they make a clay-like additive to fix cooling system leaks?

 

GM specifies that leak pellets be included when refilling the coolant system. Probably what you found in the reservoir.

 

Or like you mentioned, maybe someone used some stop leak to hide a head gasket issue. I've seen brands that are designed to provide a short term patch for a failed head gasket.