Mrs Ann . . . . .dubl'ya
It's going to be hard to just toss a weighty 50 inch Plasma set anywhere.
The most needed initial info was the analysis on your part of the source of the origin of the offending sound within the internals of the unit.
With you saying that it was NOT affecting the picture . . .that clears a LOT of the set. If not originating within the sound section, and your confirming that by turning the sound completely off, that would clear that aspect, excluding the audio output stages.
Excepting the fact that noise within those stage could pass to the speaker with the volume set to minimum.
To confirm that, an ear to each speaker will confirm.
OVERLOOKED . . . . .so far
That TV's PLASMA technology is one POWER hog, and if you will look at the units back specifications sticker, you will see it using many-many-many-many amps of power consumption. Possibly, only being challenged by the now cast aside DLP technology "hot boxes", with their $200-400 HID lamp replacement every 3 or so years..
Sooooooooo . . . . they run HOT, therefore, within those units you will find at least 4 and sometimes up to 6 small cooling fans. . . . .usually lined up across the top.
At just about the time frame of use that you are now experiencing on that set of yours , usually a fan will start having a choppy intermittent bearing seize . . . be it just for an instant . . . . . and its accompanying brief sound, not too much later it will lock down and stop working, and put the cooling task upon the remaining units. Then . . . . . overheated modules fail.
After you have cleared the " noise " as not being speaker / sound derived, turn the sound down and then with an absence of any sound.
Inspect to see that all fans are spinning and not having one locked down, ( the prior noisy one) and also listen up long enough time to hear that "offending " noise and see if it can be pinpointed as originating from a fan.
( Usually the sets sound, initially masks these noises until they get loud and frequent. )
Removing 2-4 screws and a plug IS within the capabilities of a layman in replacing a fan (s)..
Taking this units service manual, which I have as being a 26 Meg RAR and zipped file and then successfully extracting ALL of its files to get 341 pages with 48 pages of schematics and then being able to use mental retention acumen to be darting between adjunct sections for analysis,on a computer screen, to enact a down to component level repair . . . . . I think (KNOW) that a layman would have a rats anal orfice chance in hades . . . .in even starting to achieve that task.
PRAY for fan noise origin . . . . . as that big set still has life left in it . . . . and the electric company will just LOVE you and KEEP you on the top of their valued customer list..
Thassssssit . . . . .
73's de Edd