Third c450 is in my room, and I'm happy to report that the buzz/hum is verrry minimal - exactly the volume/pitch of my first c450, and significantly less than my 2nd c450. IY02 again here - confirms there is NO panel lottery going on.
1) I was getting worried as my local HHGregg manager claimed he couldn't continue exchanging new sets for others in my situation (this was my 3rd after all). He even claimed someone earlier in the day exchanged the c450 due to a "hum".....Glad this worked out. PQ on this set is excellent, and while they were getting the other TV from the back I decided to take a gander at options 2 and 3. The Panasonic S2 and the LG pj350...came away preferring the PQ of the c450 in numerous areas.
2) HHGregg upped the price of the 42" instore by 40 bucks - equal in price to that of BB and Sears for the same size/model. Bought it at just the right time a few weeks ago and with the extra $50 off coupon too.
EDIT: Upon close inspection, realized that the hum from the TV is lessened even more when the pic is in "Standard" mode vs my personal pref. of Movie mode. Std. mode seems brighter to me than Movie mode (even tho Std. apparently reaches new energy star stds), but color accuracy is off and its so bright and overexposed that it looks similar to my LCD in this mode. If anyone out there is stuck with a loud buzzer, you could try getting used to the brighter, cooler Standard mode as it may lessen the buzz.