A video of Sydney Sweeney has gone viral that appears to show her caught on camera, letting out a squeaker while working on "Euphoria" ... but TMZ has learned it ain't legit.

Here's the deal ... a short clip of the bombshell superstar is spreading like wildfire, appearing to show her ready to film the popular HBO series ... and it sure sounds like she passes a little gas as "action" is called. There's been heated debate about whether it's real.

A source with direct knowledge tells TMZ -- the clip is a fake -- kinda -- the behind-the-scenes footage is real, but the sound effect was added by some clown looking for clicks ... and it very much worked. One version of the clip had over a million views.

It's not like this is the first BS that Sydney has had to deal with while working on this season of "Euphoria" ... TMZ reported last month an alleged feud between her and Zendaya, all tied to a BTS snap, was nonsense.

Bottom line ... anyone who thought the gas clip or feud was legit, can blow it out their ... you-know-what.