why does my mac's fan make noises once I open safari?

Hey there!

Seems like Safari is using more CPU. You can find the usage in Activity Monitor. You can try resetting SMC first. This will resolve the issue with power supply.

About fans and fan noise in your Apple product - https://support.apple.com/HT202179

Reset the System Management Controller (SMC) for Intel-based Mac - https://support.apple.com/HT201295

If that doesn't resolve the issue, check if the issue is appearing in another user and safe mode. Based on that, you will need to either reinstall Mac or check for user preferences and third party apps/extensions that might be impacting Safari.

Set up users, guests and groups on Mac - https://support.apple.com/guide/mac-help/mtusr001/mac

Start up your Mac in safe mode - https://support.apple.com/guide/mac-help/mh21245/mac

If nothing works, visit the nearest Apple store for inspection.

How may other Application are running at the same time ?? How much RAM does this computer have ?? How much empty space is Empty Space on this computer. Should always have at least 15 % I=of the total capacity of the drive empty for the computer and OS to function without issues.

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation. The Built-in Security of Big Sur is all that is required.

Commercial VPNs should only be used if Mandated by Corporate Head Office for working remotely. Otherwise - ones Privacy and Security touted by the VPN is suspect.