Floor lamp, at low brightness, causes issue with a SUA1500?

[BillP]

I have a 15A circuit to which is connect a floor lamp, with two 72W Halogen bulbs (new type), and two SUA1500. When the floor lamp is set to max brightness there's no issue. When I turn the lamp to its lower brightness setting one of my APC units will make an odd noise. I can't really describe the noise other than it's similar to a constant low level static. If I turn the floor lamp to its full brightness setting the noise immediately goes away. I can repeat it every time. Any idea what could be going on?

This issue is NOT the same noise as posted here: https://community.se.com/t5/APC-UPS-Data-Center-Enterprise/bd-p/datacenter-forum

Thank you.