Don't do up the hose clamps too tightly, especially the one round the throttle body.
Remember, on the focus, the factory Ford throttle body is a crappy piece of plastic, and you can easily apply too much torque to the hose clamp and crack it.
The whole induction setup on the Focus comes out very easily, one minute max. Just check connections, the one that is commonly left incorrectly fitted is the ducting from the front of the grill to the airbox, under the battery, at the airbox end. The airbox just pushes in place, and if the ducting isn't correctly fitted, you'll get problems there. The ducting just clips into place at the airbox end also (yes, the car is a snap together toy).
I never really noticed an induction noise with my K+N until I fitted a pod filter setup (with lots of induction noise), and then went back to the K+N panel filter for a day to test the changes, and still noticed a noise.
So long as you haven't cracked anything anywhere (like the throttle body,
MAF fitting etc), and its all pushed in place (including the positive crankcase ventilation pipe - the one from your rocker cover to the bottom of the airbox), then it's all you can really check to be sure it's working ok.
From your description, it sounds exactly like my K+N panel drop in setup. As you say, if it was over the whole rev range I'd guess leak, but as you have described it, it's normal in my opinion (from your description, my experience, and not having seen it).
Tim