A major component of fan noise is added by flow through the grille - especially the pattern of stamped slots or small punched holes found on most cases. Cutting out a single opening will make a major difference. With the front fans, hidden behind a plastic bezel, a guard is not necessary, otherwise a wire grille can be fitted to protect fingers, curious cats, etc.
Another thing you can do is make the panel cut-out the same size and shape as the fan throat opening, rather than a simple circle. You'll see the throat has a curved "wasp-waist" profile, known as a venturi, which optimises air flow. Extending the venturi profile through the panel wall, blending it in by chamfering the cut-out edge, reduces turbulence, noise and flow losses due to drag and obstruction at the cut-out. Picture (right) shows Panaflo's recommendations for their 80mm case fans. Note the shape is slightly different depending on whether the fan is mounted sucking or blowing.
Most fan-maker's publish their own cut-out templates for each fan model range, so visit their site and search out the ones for your fans.
Fan noise is often amplified by the case panel on which it is mounted. Noise can be reduced by isolating the fan body from the metal sheet. Various methods can be used, such as adhesive foam draught excluder tape (useful when the fan fits in a plastic box which is clipped into place, as on my old Enlight case) or silicone rubber sealant (for semi-permanent mounting), but rubber grommets have some advantages.
It's easy to remove or change the fan, and there's no waiting for glue to dry.
Look for grommets with a 4mm bore. You'll need to enlarge the case holes to suit, then fit 4 grommets, working the rubber lips neatly round the hole.
Don't over-tighten the fan bolts, just enough to lightly compress the grommet. A drop of threadlock (or nail-varnish) will stop the nuts coming loose.
Another method described by Bluefront uses a short length of thick-walled rubber tube acting as a soft washer between fan and case (or a smaller grommet that fits the existing chassis hole could be used), the fan being attached by a couple of cable ties per hole, one acting as "bolt", the other slid up it to act as a "nut".
A commercial method is the rubber mounts made by Nexus et al.