....Crunchy Honey-combe centre :)
From the depths of my seedy past in the Design Office..... the rudder like many of the control surfaces and fairings on AI and BOE aircraft are constructed from a honeycombe-aluminium/CFRP composite.
Essentially its a stiff lattice type material that is expanded into a honeycombe structure and shaped as dsesired. (on some components the honeycombe is made from Aluminium alloy) Then shrouded in either alloy or a fibre-glass/composite skin to finish the component.
Any "foam" interior you may have seen might be adhesive or more likley, a result of a repair carried out to the component previously. When any damage is found the usual repair involves cutting out the damaged skin and a portion of the honeycombe centre - then filling with an epox resin and foam combination finally applying a patch to the skin by curing (if composite).
This method of construction results in a higher weight to strength ratio compared to traditional construction and is successfully used on numerous civil aircraft types. One issue is they are prone to insideous Ice damage after relatively light impacts (from gravel or FOD) as water craftily tries to delaminate the composite from the inside..... although I'm pretty sure the same would be true of Dog biscuits!
Ultimately they contibute to fuel savings (less weight) and reduced maintenance (reduced corrosion risk) with no impact (that I'm aware of) to the actual aerodynamic/handling characteristics.