LG-G5 here, specifically the LGLS992, and I'm on Android 7.0.
I've only had this phone for a few months, but this issue has been driving me slightly batty - mostly because whenever I try to Google for solutions I just get a billion threads on getting your vibration notifications to be stronger, or occur less often.
Sitting cross-legged on my bed, using my laptop - which is on an slide-under adjustable table - I often set my phone on my ankle when I'm not using it. And every so often, while it's sitting on my ankle, I feel this little whirring sensation coming from the back of the phone.
I liken it to a cooling fan in a laptop, just randomly kicking on, randomly kicking off, with only the slightest physical sensation if you're touching the case. But the last time I checked, no one was putting cooling fans in the back of cell phones.
Yes, my normal vibration notifications all work normally. No, I do not ever use Snapchat. My speakers appear to be fine - awesome even, compared to my previous G3.
But I swear there's no way it's not happening. Because I know what phantom vibrations feel like. Sometimes, if I haven't gotten enough sleep - which is compromised by my apnea even when I do get enough - I will get the sensation that the floor is shaking beneath my bed. Like a really low grade earth-quake. Apparently it's some kind of physical anxiety by way of sleep deprivation thing. Though it might also be a fibromyalgia thing, who the hell knows.
The point is that, because I have these other 'fake' sensations, I assumed when I first felt the whirring from my phone that it wasn't really happening, it was just some annoying new form of restless leg syndrome. So I tested it regularly and repeatedly. When the phone is whirring, the sensation is only on a very small and specific area of my ankle, directly under the center of the phone. The feeling never occurs when the phone isn't sitting there. If I pick up the phone by it's sides, it's not really noticeable. I have to be touching the center of the phone to feel it - and the sensation is stronger in the back of the phone than in the front. I pick up the phone and put it back down, the sensation is usually still happening when I put it back down - though as I said, it does shut off and on, so it's not always still whirring if I put it back down. But if I move the phone to a different spot on my ankle, the feeling is recurring again in the center of the phone.
I don't get what could be causing this. Considering the limited hardware that is in a smart phone, there's just no good explanation for a sense of whirring or very light vibration coming from inside a phone. But inexplicable or not, it's definitely not just in my imagination.