Matt,
too much sideplay in the leading driver set will allow the inner face of the leading drive wheel to contact the chassis. This will give the squeak. My fix is to place a thin washer on the axle on the inner side of each leading driver wheelset. This will stop the wheel inner face contacting the chassis on curves. I had one that would do it even on 10-foot diameter curves, until cured.
too much sideplay in the leading driver set will allow the inner face of the leading drive wheel to contact the chassis. This will give the squeak. My fix is to place a thin washer on the axle on the inner side of each leading driver wheelset. This will stop the wheel inner face contacting the chassis on curves. I had one that would do it even on 10-foot diameter curves, until cured.