Sludge in radiator with bubbling water sound in heater

When I filled up the radiator to full the reservoir drained and after driving a block I noticed a sound of water in the dash. When I accelerated from. A stop is sounded like water and bubbles rushing back against the dash. The heater had a smell strange at first unless I got used to the smell. Then the heat started filling the car with fog. The window fogged up. Next day I figured I'd check the radiator again and it was caked with a brown sludge under the cap. Lots of it. Reading online saying it could be bad Dex coolant gone bad and turned to sludge. I have heard it can be a head gasket. A cracked block. I have heard it could be coolant leaking into the transmission cooler. I don't. Transmission fluid in the radiator. Doesn't overheat. Took it to the dealer and he said hes going to have his mechanic look at it but i'm lost as to what to do.