Thank you all for your suggestions and input. I've created a webpage (forgive the cheesy googlepages formatting) with some data, i.e., audio files and spectrograms of the noise, the address is at the bottom of the post. After looking at the spectrogram, I am fairly confident this a 60 cycle impulse noise of some sort.
I did take the rig out to my car, drive out to the middle of nowhere and do some testing (unfortunately no laptop along for empirical analysis). When transmitting, I did notice a similar type of strange signal appear, however it showed up in the way of an increase on the S Meter, not an audible noise. That is, the S meter was showing 4 or 5 background noise, but increased to 7 (this time with no really noticeable audio change) following transmission. Application of the NB brought the S meter back down to 4 or 5, but again no readily noticeable change in audio.
I have repeated these experiments with and without the antenna tuner, with the same results, so I do not think that is the cause.
In terms of the exactly location of the noise on the band. It appears to focus on the frequency on which I transmit, initially covering almost the entire band. Overtime though (maybe 20 minutes), the center of the signal appears to move up in frequency until it finally disappears.
So, any further input would be greatly appreciated, and thank you all for getting back to me. Sorry about the delay in my follow up post.http://iewalsh.googlepages.com/ft-897transmissionhum