I found a lot of Federal Government radio scanner frequencies on the Federal Government Wiki.I also tried to limit search between 406-420 and whenever the seach would stop on a frequency there was only a rush of air or at least that's what it sounded like-no voice what could be causing that?
Most likely you found DES encrypted signals. DES encryption sounds just like white noise which is what you are describing.
What are you using for an antenna?
Run your searches with a standard unamplified antenna for starters.
Encrypted P25 and DES sound totally different to the ear.
edit: being as you said you found a LOT of this white noise sound makes me think you may be using an amplified antenna. Or your squelch is set too low. What radio are you using and does it have some type of signal meter indication?
If you have a signal meter in the scanners display, what does it show when the search stops on the white noise sound?