Q: engine makes a rattle noise on cold start. Noise goes away as it gets warm. When i turn on the ac the rattling noise is louder.

Among the possible sources of this noise are accessory drives (pulleys, alternator, etc.), the timing belt (or serpentine belt) tensioner, and loose heat shields. The fact that use of the a/c worsens the noise would point toward the serpentine belt circuit of components but that is speculative until the car is actually inspected. If you desire a resolution to this, the recommended service is a vehicle noise diagnostic. If you request that service, the responding certified mechanic will definitely find the origin of the noise and let you know of the recommended repairs. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.