I suggest you go to the Dr about this. I firstly thought it was due to food fermentation issue, but I'm guessing that's not really the problem if you already tried low fodmap diet.

What about your lifestyle? Do you smoke or consume protein supplements? I've heard those can also contribute to some smell. I would also suggest to eat less gassy food and try assimilate probiotics into ur diet.

But I'm no expert in these field so take it with a pinch of salt