LG G3 Cat.6 — Random Vibration on LG G3 (Sometimes)

I just got an LG G3 last Friday and I've noticed twice now that it has randomly vibrated in succession- with no notifications. It doesn't happen often enough to diagnose the problem myself, so I tried to find something on it but nothing has helped/been helpful. My phone is always on 'Vibrate', but the problem is that no notifications show up. The last two times it has done this, it's been on low-ish battery (30s-40s), but not low enough to get the battery saver notification. I also know it isn't "phantom vibrating" or whatever, because I can hear it vibrate on my desk (out of pocket). If someone could help, that'd be fantastic. Thank you.

P.S.- Would this be a software issue or a hardware issue (basically, is this fixable)?

So is there a way I could fix this?