Re: rear suspension - yep, i've noticed the odd 'skip' from the rear, usually at highway speeds with a turn involved. Torsion-beam suspension i think...cheaper than fully independent.
Tyres - these are the biggest change you can make to road noise - and as others have said I think the Goodyears / Michelin's (on the 110) get noisy when worn. I notice it on the rough broke stuff here in the UK after 15K miles. The car was practically silent in France...typical! There again they pay through the nose for the Peages. I'm planning on changing the tyre brand when these ones need replacing in about 10K miles.
I wouldn't bother with Dynomat on this car - the manufacturers spend serious time and effort tracing structure-borne noise paths on road jigs. The only way you can achieve results is to completely cover any inside panel with the stuff...and it's heavy, and expensive, so you negate the whole point of the lightweight approach that Citroen took.
I looked into active-noise cancellation kits a while back, but they also need proper test and analysis to position the mics correctly.
Best bet on the Cactus is to get the Arkansys amp package and crank the tunes up!!!
Wheels, Tyres, Suspension, Chassis