i know it's just 'wrong' to return an f8500 just to buy another f8500 at a higher price, but i think it's equally 'wrong' to return an f8500 because of buzzing, just to buy another tv that looks terrible by comparison.
unfortunately you're now in a worst-case scenario situation. every choice you make is going to give you regrets. the only thing i can suggest is trying another plasma while you still can. especially if it's a straight refund and new purchase, you'll get a brand new return window. led isn't going anywhere, it's only going to get cheaper or better, and the same with oled, but plasma IS leaving the market, and anyday could be the last day you can buy one at a reasonable price. so buy now, think later, return if needed.
buy an f5300 or h5000, continue looking for f8500's. if the f5300 or h5000 buzz as well, maybe you're just too sensitive to it, and no plasma will work for you. but then at least you KNOW that. and in the mean time, if you happen along another f8500, buy it. you can always return the f5300/h5000.
basically, what i'm saying is that you only need to rush if you want another plasma. rushing into an led at this point is silly. even if that's what you eventually end up with, do it right, research the crap out of it and view as many as possible with your own eyes. shopping for a good lcd is SO much harder because the market is totally flooded with them, way more competition.
Displays: Samsung PN64F8500/JVC X35
AVR: Pioneer VSX-1018AH, 5.1 audio
Sources: HTPC(Mediabrowser), PS3, XBOX360, Wii, Sony DVP-CX995V
Control: Harmony One