What sort of noise: clicks, squeals, whirrrr..?
Could be a faulty fan but more likely just some inbalance until the fan gets up to full speed.
Maybe the blades are touching the housing or a cable and once full air flow is established the case and/or blades flex ending contact.
Power down, unplug, open the case. Use a bright flashlight to inspect the fan: Housing, blades, and surrounding area. Look for signs of contact or damage. Maybe some piece of debris in the fan.
Try gently wiggling or twisting the fan. Does the entire fan housing move in any manner?
Ensure that the screws are all evenly snug and tight. Do not over tighten.