One beep is a RAM error that the POST diagnostics failed. While it may appear fine after booting the issue is a flaky RAM. Best to try a different set of SO-DIMM's to see if that fixes the issue. It could be some corrosion on one of the pins or the socket frame is not holding the SO-DIMM firmly.
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I would see if diagnostics caught any errors if you can get it to run Apple messed up the firmware and never fixed it. Haven't tried any third party tools and they often run a walking program flipping different RAM registers which does take a bit of time (more with larger SO-DIMMs or multiple present.
One beep is a RAM error that the POST diagnostics failed. While it may appear fine after booting the issue is a flaky RAM. Best to try a different set of SO-DIMM's to see if that fixes the issue. It could be some corrosion on one of the pins or the socket frame is not holding the SO-DIMM firmly.