Oh man I went through this a few years ago…though I doubt it’s the same problem.

I was a custom integrator and one of my clients had a beeping UPS. Unstable incoming voltage. Ok, not the first one of these to kick the bucket so I do the RMA.

Later that same week I’m at a different client for the same thing. Call my boss and tell him we should start looking at other manufacturers, he agrees.

RMA arrived and I install it. Later that day the client calls…the beeping is back. I call my boss and he’s at yet another house dealing with the same thing.

“Wait…you’re at [client]'s? That’s a few blocks away. And so are [other 2 clients]. Are we sure it’s the UPS…?”

TURNS OUT IT WASN’T! There was something fucky with the neighborhood grid (the transformer? node? I don’t know, I’m not an electrician). We talked to ComEd, they and a few weeks later they did some repairs. No more problems.