This seems like a never ending issue with AT&T Arris modems, an issue that at least reported here, here and here. I previously had an Arris modem (not sure which modem) with Sonic (www.sonic.com) which under the hood uses the AT&T network to provide internet services and the AT&T-branded Arris modem always made a high-pitch noise.
I am now with AT&T proper and again the Arris NVG599 modem they provided makes this awful high-pitched noise (and I'm in a completely separate home).
Let me get you to the right place to get this issue with the modem resolved. This tool called Troubleshoot & Resolve your AT&T modem. This tool is both instructional and informational. Under each title you will see in light blue, Show more> tap on this and it will expand providing you with detail instructional and information on the process. I am also providing you with a link for additional resources.
May your day be filled with small miracles!
Georgia, AT&T Community Specialist
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