A great thread thanks. More than I expected.
Good advice all round.
I could rerecord it and the above advice is great for that too, but I always like seeing what I can do with what I have as a learning opportunity.
In this case I got some pretty impressive results (IMHO) out of the sonitus gate with the low pass filter.

On mono tracks, where you're wanting compression, consider multiband compression (compress the noisy frequencies less or not at all). Also consider lowering gain on noisy frequencies.

What about plain old EQ. You can automate some of the EQs. I think the Pro Channel EQ Automation is a little goofy or slow or something. Sometimes the automation works and sometimes it's slow. Like a tight millisecond notch may or may not work on bouncing in my experience. You might consider adding another EQ for automation. I've noticed some threads on PC EQ automation as well. Not sure what the truth is.

But certainly you can automate the other EQs you can add to the project. That way, you can duck the noise when you boost the quiet parts and relax the EQ when you have more signal.

R-Mix works well for this.  I just used it to clean up some old analog tape recordings for a friend.  It actually has a preset for hiss which dialed it in very close 

ive found the best restoration/remove hiss, hum, exc is izotope RX 2.
there may be a 30 day trial im not sure but if you happen to get your hands on this tool it is amazing.
ive taken distorted crummy 25 year old reel / 4 track vocal tracks and cleared them right up.

good luck buddy.

 This is truly hiss in the source, and not as a result of a plug-in, right?

Steve- Wave Lab7 Elements is only $90. It comes with everything you need for home studio and semi pro work. Your Sonar plug ins will also be available. The full blown version would be nice but is $$$. Once you've used it, you will find using Sonar to edit a wave file or mastering pre historic. It's the same with Melodyne for pitch correction, simply better for the job.