Vibration damping products like Dynamat are great on sheet metal that vibrates and makes the tinny sound you want to kill in a car. What kind of filter are you talking about? I don't think there's anything you could do for a HOB
, for example, being out in the open.
But if a canister in a cabinet is a bit noisy you could add sound absorbing and/or sound blocking materials to the inside cabinet walls. You could also put it on a vibration absorbing pad so the hum doesn't transmit to the surface it's sitting on.
A good site that covers the difference between vibration damping (CLD), sound absorption (CCF) & sound blocking (MLV): Sound Deadener Showdown
Another great site: Cascade Audio Engineering - Sound Damping and Sound Blocking