If it goes away when the brakes are depressed (even slightly), it sounds like caliper pad squeal is the issue. They make a goop that can be applied between the pad and the caliper to lessen/eliminate the sound.
However, it's odd for such a noise to start up unless brake work was done recently. Is that the case?
1992 XJ 2" lift -- RIP
2000 XJ 3" lift -- Australian Ford Big Brakes up front, rock rails, ZJ rear discs (with WORKING parking brake), C4x4 rear bumper/tire carrier, Hella H4 headlights, IPF driving lights, OEM skid plates, OME springs/shocks, JKS LCA's, JKS Sway Bar links.