Battery only charging until 60 -70%

That's interesting - maybe someone else will correct me but I'm pretty sure the buzz should only happen when the charge cable is first connected - I've got Bangles I leave on charge all the time here and I don't hear them buzzing.

So maybe the issue could be the USB charger that the charge cable is plugged into? The Bangle doesn't charge with a lot of power so maybe the charger gets confused and turns itself off/on thinking nothing is connected?

Is there an option to switch off the beeps?

There's no option but the buzz is built into the battery widget: https://github.com/espruino/BangleApps/blob/master/apps/widbat/widget.js#L3

So you can either uninstall the widget completely, or install one that has an option like widbatpc, or one that doesn't buzz at all like widbata or widbatc (I think)