2006 Volkswagen Jetta 2006 TDI Making Noises

Recently I cranked my 2006 VW Jetta TDI and noticed a “popping" coming from under the hood and the engine was running rough. It sounded as if a fan blade was intermittently hitting the shroud but the fan was not running. I pulled the covers from the top of the engine and the sound was louder. It sounded as if a piston was striking a valve, but not on every stroke. I removed the oil fill cap and there was a significant pressure release. At this point I turned the engine off and it still sits in the church parking lot. There was no smoke from the tail pipe and no puddles under the car. Is there the possibility that a valve retainer came off or a spring broke? If so what damage could I expect? What would be my next action? Is there a positive crankcase valve that may be malfunctioning? I do not plan on driving it anywhere. I have a tow vehicle and can rent a trailer to either bring it home or take it to a repair shop depending on what is wrong with it. I need to go the least expensive route as I also just lost my job and have limited resources.