2000 2.5L V6 Avenger noise at start up.

When I start it there is a knock that goes away after 2 or 3 minutes, it can be heard when throttle is applied during this time. No noise is heard at idle or after rpm's go up. I'm thinking something in the exhaust that expands when it heats up after a few minutes, any ideas?

Piston slap. The 3.0 V6 have this problem. Once the piston heats up and expands some, it gets less but does not go away. Also crap lifters that tick.