Yes; I did try another filtered UPS. We did not install this KSU; someone else did. And it's been working well since 97'. This noise started back in Nov. and there has been no changes in wiring in the recent past to explain the inductance theory. We are a small outfit and are phasing out Panasonic equipment; i.e..no back-up cabinets. I've has several experiences in the past where the keyset was actually making a similar noise and resetting the tel. cleared the noise. This noise is moving around?? The women that work in this customers office swear it's the building ghosts. I'm beginning to believe them.