Radiator flushing
I grew up in the auto repair business,we had a radiator shop who rented from us,the old guy told me that a lot of times it was not rust ,it was casting mud being released from the casting, he said that after engines were cast,they were treated to a sealing process which breaks down over time.If the "rust" feels slimey it is mud . I can remember digging mud out of engines when changing core plugs. When I was in trades school the old professor would correct us when we called them "freeze plugs", he explained that the only reason they were there was to remove the casting mud .Kenny