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 like 1 or 2 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 and then start the car, 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 recommendations??

I've noticed something similar with my 2006 S40 2.4i It's a very brief sound right at start-up. After a few seconds it goes away. I'm using Mobil 1 10W-30 in my car.

I've also noticed new sound coming from under the hood now that I have moved from Florida to Tennesee. When it is cold outside in the mornings (21-30 degrees F) I have noticed a terrible rattling coming from under the hood after the initial start-up. At first I thought this was engine valve noise but I finally got out one morning and opened the hood to investigate.

I discovered that if I push on the air breather housing this noice stops completely. The cold temps. must have a contraction/expansion effect on the plastic air breather housing that causes this. After the engine warms up or I rev it past the idle speed it stops.

Go figure!

Chino, what octane gas are you using? Typically when an engine is knocking, it is because the octane rating is too low. Usually it is a sustained knock though, not just a brief one like you describe. I'd say try filling up with premium gas and see if it goes away. Just make sure that you are on empty before you fill up, or you will just end up mixing the low/high octane and making mid-grade. If that doesn't work, or if you are already using premium, I'd say to take it to the dealer.

Good morning guys.
It doesn't help resolve the situation much.........but my 2004 30000 mile 2.4i does this too......it's a bit annoying, and definitely not cool, but I learnt to ignore it.
My car has been through AA check and regular services and other than this brief startup rattle, it runs fine.

I've always been using 93 octance gas. So I know the gas is not the problem. I also learned to ignore the brief knock as well. The car still runs good.