I have a 2008 volvo c30 t5 with 124k miles and after a bit of hard driving in the mountains it developed a knock. The knock was tied to the rpms and only present after 2k rpms under load. Definitely sounded like rod knock. Now after sitting for a night it's completely gone. What the hell should I look into? I had no check engine lights, and the oil is new Pennzoil 5w-30 full synthetic with no sparkles or bubbles in it.