My Ionic Breeze GP had the hissing problem from time to time. When I first got it, I even sent it to have it replaced and the replacement unit also started hissing. Therefore I think it is a pretty common problem.

Cleaning it as described in the product's documentation and in other Fixya solutions such as this one:

http://www.fixya.com/support/t154553-ionic_pro_sizzles_crackles_when_turn

seems to help a lot of the time. Also, more frequent cleaning (don't let it get too dirty or wait until the light comes on) seems to help. However, I've had time where no amount of cleaning, blowing air, claning the wires, etc. will make it quiet.

That brings me to my point:

I recently had to take the whole unit apart after it totally stopped functioning. This had nothing to do with cleaning, but was a bad switch (my posts regarding this fix are also on Fixya). What may be relevant is that when I had the unit apart, not only did everything get a good cleaning, but I also found several dead insects that were down in the bottom part, down by the power supply components, down where you would not normally clean because that area would be behind plastic covers.

After removing the insects and blowing that area out with a can of air and dusting with a cloth, after reassembling everything my Ionic Breeze is running the quietest it has since it was new, and when it does make noise, normal cleaning makes it go away.

What I am suggesting is that maybe some of these chronic noise problems that so many people experience are due to dead bugs or dirt beyond the area that one normally cleans.

BE CAREFUL IF YOU REMOVE THOSE COVERS. YOU DON"T WANT AN ELECTRIC SHOCK. MAKE SURE THE UNIT IS UNPLUGGED AND DON'T TOUCH ANYTHING WITH A CONDUCTOR MATERIAL OR YOUR HANDS.

When my unit makes that sizzling noise, I clean the blades but before I replace them, I blow compressed air (in the can, like for your computer keyboard) in the body, though the grids. Then I turn the whole unit upside down for the three seconds, several times. Then I replace the blades. Sometimes the noise stops right away, but sometimes its takes a while. But it stops.

Unfortunately Sharper Image has discontinued support on the product line.  There was a recall going on with the product but since the company is chapter 11 right now, I don't think it is being honored right now.  
What happened was Consumer Reports ran a test on ionizers, Sharper Image received a bad score and filed a lawsuit.  They lost the suit and had to pay all legal bills. 
EPA has reported that using ionizers in a house or business may be harmful to your health.  You can read an article about it on their website.  
The Consumer Products Safety Commission is going to release a final report on all ionizers by the end of the year.  Their decision may discontinue all sales of any ionizers in the United States.  This industry should have been regulated years ago just by the EPA data alone and for some reason it has been ignored.