Welcome to the forum.
The DeNoiser Effect is primarily designed to remove analog tape hiss.
For the removal of ambient noise from an Audio file, this article might point you to some other adjustments, and especially to Adobe Audition: http://forums.adobe.com/message/2860521#2860521
Good luck, and hope that helps,
I'd agree with Bill. The effect in PP just isn't very good. Do this kind of thing in a real audio program.
Though I have never seen a report from Adobe, it appears that the DeNoiser Effect needs a second, or two, to "lock in," and define the "hiss." There have been several threads (one current), where the Effect left hiss untouched for almost 2 sec.
Audion would be the route that I would go.
If the noisy clips are the only thing on that audio track, you can add the Denoiser effect from the audio mixer panel rather that applying it to each clip. The effect will apply to everything on that audio track, and I've found that it gets rid of the delays and oscillations of the Denoiser effect lots of times.
Open the audio mixer panel and you should see a slider there for each track and a master. At the top left of the mixer panel UI is a triangle toggle like the ones on all of the bins in Premiere's UI. Use the toggle to expand the mixer panel to show the fx bus. There you can pull down a list of all of the audio fx to apply to each track. After adding an effect to a track, right click on it and choose "edit" to make adjustments. You can do this while your timeline is playing to hear the results. You can also keyframe effects on the timeline by setting the track display to "show track keyframes". That option is down on the audio tracks in the timeline panel.
Since I learned of these options, I know find them indispensable and you them on nearly every project