The truth was that Sam's Dean had taught him how to fix the car, and most recently they had rebuilt the Mustang...which was in the next garage over. Sam had artfully taken off a few belts, disconnected the carberator and mismatched some spark plugs. The whole thing looked like a heinous mess and he was quite proud of himself.
"There she is. I uh, well it was making this funny sort of rattle so I tried to...you know." Sam gestured in a helpless manner at the car. "Think you can fix her?" He took a long pull off his beer, before stashing the rest of them in the fridge in the garage. He then pulled up Dean's favorite playlist from the MP3 player and let the mullet rock fly. He'd start artfully prying as soon as Dean was engrossed in car fixing.