You have to use a locator devise, anything from a broom handle to a stethoscope. You have to isolate where it's coming from.
P.S., I can't tell you how many times I have found a sound like that is either timing or exhaust/headers leaking or both. Sounds exactly like something thocking/knocking or ticking. I'll bet you my piggy bank that's what it is. Whatever it is, goes away after warm up because of expansion.
Your motor may have a detonation sensor/knock sensor. If it fails, motor may detonate and make that sound.
http://www.oreillyauto.com/site/c/d...nation+sensor&pt=02200&ppt=C2179
However, the symptoms for that are knock under load or highway speeds.
At idle like yours: My guess, Exhaust leak.

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