ES 550 Let my MAC FRY

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3 year old ES550, replaced battery 3 months ago when the unit started making a weird sound. The alarm buzzer randomly produces a weak grumbling sound for about 20 seconds then stops. The connected equipment is unaffected and stays operational. This ritual has been happening for a month or so. Well this morning I hear the same grumbling but it keeps going. I go in to my office and see my 27” MAC and my cloud server are both off. Long story short my MAC will not turn on and the power supply for my cloud server is fried. Both are connected to the battery plugs on the ES550. 

How can this happen? How can the UPS cause my equipment to fry? I have a BVN650 on the same 120vac circuit, sitting right next to the ES550, and it hasn’t made a sound. There was no power bump or outage, it just fried my equipment. Comments will be appreciated. 

PS: The UPS was monitored 24-7 via Powerchute and it never indicated any issues with the unit.