# Knocking engine **URL:** \ **Category:** Maintenance/Repairs\ **Tags:** forester, subaru\ **Created:** [January 6, 2011, 9:51pm UTC](https://community.cartalk.com/t/knocking-engine/59637 "2011-01-06T21:51:49Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![IdahoRudy](https://avatars.discourse-cdn.com/v4/letter/i/c57346/32.png) [@IdahoRudy](https://community.cartalk.com/u/IdahoRudy)\ **Post date:** [January 6, 2011, 9:51pm UTC](https://community.cartalk.com/t/knocking-engine/59637/1 "2011-01-06T21:51:49Z")
What causes knocking in the engine of my 2003 Forester when it is cold? Is it hamful to the engine? ---
**Author:** ![lion9car](https://avatars.discourse-cdn.com/v4/letter/l/59ef9b/32.png) [@lion9car](https://community.cartalk.com/u/lion9car)\ **Post date:** [January 6, 2011, 10:15pm UTC](https://community.cartalk.com/t/knocking-engine/59637/2 "2011-01-06T22:15:58Z")
Is your oil level correct? Has it ever been run low? Has all maintenance been done on schedule? Are you using the required gasoline octane? ---
**Author:** ![kawasaga](https://avatars.discourse-cdn.com/v4/letter/k/2acd7d/32.png) [@kawasaga](https://community.cartalk.com/u/kawasaga)\ **Post date:** [January 6, 2011, 10:27pm UTC](https://community.cartalk.com/t/knocking-engine/59637/3 "2011-01-06T22:27:37Z")
Those engines are known to get lonely, especially when they’re cold. Next time it knocks, ask it if it want to come in. ---
**Author:** ![VDCdriver](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/vdcdriver/32/30_2.png) [@VDCdriver](https://community.cartalk.com/u/VDCdriver)\ **Post date:** [January 6, 2011, 10:51pm UTC](https://community.cartalk.com/t/knocking-engine/59637/4 "2011-01-06T22:51:38Z")
It is very possible that you are simply hearing piston slap, which–supposedly–is not harmful. I would suggest that you park your car at the mechanic’s shop overnight, and go back in the AM to start it while the mechanic listens. If he confirms that it is piston slap, then there is probably nothing to be concerned about. If it is not piston slap, then start saving some money for repairs. ---
**Author:** ![the\_same\_mountainbike](https://avatars.discourse-cdn.com/v4/letter/t/c89c15/32.png) [@the\_same\_mountainbike](https://community.cartalk.com/u/the_same_mountainbike)\ **Post date:** [January 6, 2011, 10:52pm UTC](https://community.cartalk.com/t/knocking-engine/59637/5 "2011-01-06T22:52:34Z")
All knocking is caused by shock waves, and until one determines exactly what’s causing it, the knocking should be assumed to be potentially harmful. The answers to Lions’ questions will be important clues to finding out the source of the knocking. I would add where you live as a clue. When I lived in North Dakota I found it took a lot longer at subzero temperatures for everything to start working normally. ---
**Author:** ![ok4450](https://avatars.discourse-cdn.com/v4/letter/o/e9bcb4/32.png) [@ok4450](https://community.cartalk.com/u/ok4450)\ **Post date:** [January 6, 2011, 11:06pm UTC](https://community.cartalk.com/t/knocking-engine/59637/6 "2011-01-06T23:06:36Z")
You need to define the knocking a bit better. Present all the time, goes away in 10 seconds or does it take 20 minutes, etc. ---
**Author:** ![kawasaga](https://avatars.discourse-cdn.com/v4/letter/k/2acd7d/32.png) [@kawasaga](https://community.cartalk.com/u/kawasaga)\ **Post date:** [January 8, 2011, 3:14pm UTC](https://community.cartalk.com/t/knocking-engine/59637/7 "2011-01-08T15:14:44Z")
Oh, and BTW, a GM study recently determined the cause of all engine knocks to be overly aggressive hearing aids.