The noise is more of a mechanically transmitted one to places that will resonate to it. Thus, the addition of bubble pack won't really cure anything. The addition of foam firmly glued to the underside will probably do the job as this is a mass that will dampen the resonance of the large resonating areas. I've been using spline roadbed for so long that I've never considered the little noise that comes up from the railroad - mostly ticking noises from railjoints and rumbling from the rolling wheels - to be that bad. Occasionally when I see another layout, the noise does get to me but that is often loud poorly maintained locos or those locos which are just loud. In addition, the locos that I run are pretty quiet for two reasons, they are made quiet and they don't get run at full speed where the unbalances of the motor and so forth don't get going badly. Those loud railroads also seem to be of the more flat variety which means that the plywood central version of making a layout really is loud, even with the cork roadbed.
-- Bob May Losing weight is easy! If you ever want to lose weight, eat and drink less. Works evevery time it is tried!