is the mac mini noisy ?...i constantly hear the fan

No, it is not normal.

Try resetting the SMC > Resetting the System Management Controller

1. Shut down the computer.

2. Unplug the computer's power cord.

3. Wait fifteen seconds.

4. Attach the computer's power cord.

5. Wait five seconds, then press the power button to turn on the computer.

Then if the fan is still loud, return it to the Apple Store or Apple Authorized Retailer where you bought it and request a replacement.

Hi there Har123. Download a free app called smc fan control. and any other app to check the internal temperature of the mac. It could be a sensor malfunction and may think its over heating, or the HDD or RAM itself overheating. Use those apps to check if the fan speed is at a high rate and if the mac's temperature is too high. If both of them show these symptoms. Please show the apple store that you want a exchange. Because if you show them the proof right away, they will exchange your mac more conveniently. Rather saying oh my mac is louder than any other macs. Apple can use excuses to delay to fix the issues as they need to verify what is the problem leaving you disappointed with the service. You need to tell them the specfic causes for the specific problems you have. And everyone is a winner.