So I had my oil changed today, and put in 5w40 synthetic as I have previously, same shop as well. This evening, and part of this afternoon, my engine has developed a really loud tick at low rpm - it almost sounds like a diesel. It seems to be coming from the top of the engine. While driving, there are periods where it goes away, but so far it has always come back. Any ideas as to what could have happened / what it is?