Why am I getting short, tiny vibrations during a call (not alerts)?

I noticed his once or twice when I had an iPhone 5s before. Just got this phone, iOs 9.2.1, and I've noticed when I'm on one calls, I hear short (~100 ms) vibrations, what seemed like randomly and sporadically. Now that I'm paying more attention, I noticed one burst was about second apart, four times, then it stopped.

I really haven't noticed any common connections, until my last two calls, I was connected to my Bluetooth car stereo, but was using normally instead. I don't know if that's common to previous times though.

I've on the phone with Apple Support, and the first thing she was have me check to see if it was notifications that had been suppressed. Not the case. We hard rebooted, updated iOS, etc etc, and now I'm waiting on a return call from a specialist.

The vibrations are very short... shorter than the vibrations when you set your fingerprints on TouchID.

Anyone have any experience with this?

As a completely unrelated side note, why doesn't a mobile device giant have a mobile version of their support page? And why doesn't the question writing form function properly on iPhone? Seems silly.