Hi pedram83
Welcome to the Apple Community.
Seems there is just not one pat solution to the static / noise sit difficulty,
and
we don't know much about your own particular usage / location / various devices etc.
You might want to have a look through these suggestions, and follow through on any that seem relevant:
(1) It may help to check your software is up to date on Beats Flex, and on all devices you use them with.
Eg: About Beats Updater – Apple Support
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(2) :You might want to check that you are only connecting earphones to one device at a time
(Eg may not be switching between multiple devices.)
It may help to reset your Beats Flex earphones
Apple support's article:
Set up and use your Beats Flex earphones after service – Apple Support
says:
Reset your Beats Flex
Press and hold both the power button and the volume down button for 15 seconds.
When the LED indicator light flashes, release the buttons.
Your earphones are now reset and ready to be set up with your devices again.
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(3). If the static problem still persists:
See if this Support article can help:
Resolve Wi-Fi and Bluetooth issues caused by wireless interference - Apple Support
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(4) Sound troubleshooting – Beats headphones and earphones – Apple Support
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More support, if needed:
Beats Flex Support
https://www.beatsbydre.com/support/earbuds/beats-flex
All the best :-)