Haha, I had this same exact problem long time ago. And people back then really didn't help me or agreed that they had the same noises. But here's a fix!

This is how I fixed my whistling around mid-RPM's like you noticed. For some reason, some Hondas experience this while others don't.

To fix the whistling, you have to adjust the idle speed in your throttle body. Turn it up for more air flow at idle. I'd say turn it about a turn to a turn and a half. That should do it. If it doesn't, turn it up some more. After doing this, the sound of my intake changed into a deeper roar and the sound near redline also changed to a deeper note as well. But the most important thing was that I didn't hear whistling anymore! And I think I got more power out of it.

So! TURN YOUR IDLE SPEED ADJUST SCREW ON YOUR THROTTLE BODY TO GET RID OF YOUR WHISTLING. Do this and report back to me because I haven't seen anyone else do this and tell me that it worked. But my friend suggested it because he does this when his friends get intakes to let more air in. Also after you confirm that the whistling is gone from the idle speed screw change, reset the ECU so that the engine will breathe better.

Another note: THIS ADJUST DID NOT RAISE MY IDLE RPM'S FOR STRANGE REASON. IT JUST MADE THE INTAKE NOISE CHANGE AND ELIMINATED THE WHISTLING. I hope I made that clear. Now see if this works!