In Reply to: What's that noise? posted by bring in the da' noise on October 24, 1999 at 14:09:53:
... then scuderia is exactly right. When you turn off your engine, it cools down. Different parts may cool down at different rates, due to different specific heat capacities of the metals used (aluminium vs. steel, for example), different amounts of surface areas exposed to the cooler surrounding air, different layout, size/mass, etc. etc. etc.
As you know, when metals cool down, they contract. If different metal parts in your engine contract at different rates, this results in tension being built up. When this tension is released, this results in the *tick* *tick* sound you're hearing.
If you paid attention, then you'll notice that the ticking will be faster if you drove the car hard (i.e. the engine is very warm), or the surrounding air is particularly cold...