Question : Problem: UPS unit emits constant tone when turned on.

Had an older UPS that finally died...battery no longer holds a charge.  Bought a new APC Back-UPS ES 750.  

With nothing plugged into it, I plug it into an outlet.  When I press the on button, it blinks green 16 times and then emits a constant tone.

 I called tech support and was told that it might not yet be charged.  I have had it plugged in almost 10 hours.  I get the same result now.

I have tried it in different outlets...same result.  If I hold the power button in to force a self test, a little red light with an X on it lights up.  Any help would be greatly appreciated.

Answer : Problem: UPS unit emits constant tone when turned on.

It sounds like a faulty UPS - I would exchange it.  The tone is usually an indication that the battery is not charging, so either the battery or the charging circuit is defective.