2005 S40 Engine knocking
 

Okay so I have a 2005 S40 2.4i with 56,000 miles on it. The car engine runs and sounds perfect. When I first start the car up the engine warms up normally with no engine knock or anything. I will drive to work with no problem and park the car and after like1 or2 hours after the car has been off and I go to start the car, the engine will knock for like 2 seconds like something is loose inside the engine. After the 1 or 2 seconds of knocking the engine sounds perfectly fine and quiet and runs with no problem what so ever. Its weird though, if I park my car and let it cool down for like more then 3 or 4 hours andthen start thecar, the engine will not knock. It seems to be that only when the car engine is still warm or the oil might be to thin that is causing the knocking?? I am currently using the recommended oil 5w30 castrol syntec. SHould I try using thicker motor oil ?? Any recommendation??