Wondering about your use of 4mm (5/32"+or-) ply under the track.
I know that you work in the smaller scales, and i am sure that your equipment runs as smoothly and quietly as possible, but did you do any comparisons before settling on the ply for roadbed? Would not track fastened directly to the foam be even quieter? There is the possibility that the foam beneath the ply dampens any resonance. I just remember how noisy O scale track directly laid on 12mm (1/2") ply was. Long ago- and I tried, quite unsuccessfully, to dampen the noise by gluing Cellotex ceiling tiles underneath. After that experience, I have always used Homasote atop the ply.
____________________ Fix it again, Mr Gates--it still works!"