BE wrote:

We live in an 80+ year old house with wiring of similar vintage (I assume).
All the computers are on UPS devices. My main computer is on an APC Back-UPS
Pro 1000 with fresh batteries, which is certainly powerful enough to handle
what's plugged into it. I will say that because there are no grounded
outlets in this house (except for one in the kitchen), the APC unit's
three-prong plug gets its 100v juice thanks to an adapter that ignores the
ground prong.

My stepdaughter runs a blow dryer in her room and when she does, my APC unit
makes a humming buzz sound for as long as the dryer is on. The unit never
beeps (as it does when there's an outage or dip in power) - it just makes
this buzzing humm.

Aside from whatever might be said of the lack of grounding (which cannot be
helped), is this buzz indicating that any damage to the computer could be
occurring?

Does it only do it when the hair dryer's on low heat ?

I've come across something similar *once only ever* and it was also bad wiring.

The hairdryer uses a diode for low heat and this creates a DC content on the AC
line.

You need your wiring replaced.

Graham