Hi All, 
I solved the Strange Sound Anomaly that happens when using Noise Reduction (atleast I solved it for me) and thought I'd share... Some people say that it sounds like a flanger type effect, some a aquarium gurgling, I'd decribe it as a metalic chattering... Anyways come to find out it's not WavePad that's the problem: it's the Audio Sequencer that I was using... It was Cubase LE; and the way I found out is I was listening to something that I just recorded through a pair of Sony Studio Monitor Headphones and as I turned up the volume lo-and-behold I could hear that metalic warbling sound start to come through... So I downloaded and installed the Reaper Audio Sequencer @ http://www.cockos.com/reaper/ and problem solved... I'm now getting a very clean sound! Harah WavePad! x 3.
I too was blaming WavePad but it's not the culprit; it's my audio sequencer "Cubase LE"... Wavepad was just bringing the metalic chattering noise to the forfront when it removed the Noise... Sorry WavePad.
Also, when I use the "remove noise or hiss" prog that is listed in the left margin (also listed under "Effects" > "Noise Reduction" > "Apply Muti-band Noise Gating") I get the best results... I just click on "remove noise or hiss" set the "Noise Gate Level" to around "-26dB" and click "OK" and I get nice clean sound without getting that choppy Noise gate effect... Hope this Helps and makes you as happy as it's made me (TGBTG)!
Take care and Love You,
PeaceMaker