Okay, I know the sound you're talking about. I tried everything! I plugged in the lightening connecter. Plugged in headphones. Nothing helped this awful sound. It got a lot worse as time passed. When I 3D touched an app that didn't support it and the Taptic Engine made the 3 short taps, I never got the sound. By the time of writing this, the iPhone 6s Plus was doing it even then.
What I heard was a buzz, that while constant did have some come and go as far as loudness. It didn't start this way, but it was acquired with time. Over the course of having the phone for a little over a week. All down hill. The sound only happened while the phone was laying down, screen facing up.
My best words would be to really put your Taptic Engine to the test before you decide your new iPhone 6s Plus is for you.
Create a custom vibration tone for one of your contacts. Really lay on it. Have no mercy. I feel like if it's going to break, it will be during this time. Replay a few times, over a few days. You have 14 days I believe. If Apple sees your issues is covered (Mine was) then you have a year.
I can admit. I was amazed by the Taptic Engine. I 3D touched a lot of apps that didn't support it. The bumps, the tiny feedback taps had me in awe. Maybe I broke my iPhone with my child like appreciation for it. To be honest, it's very highly unlikely.
I do feel like this problem needs a bit more attention.
The lady I talked to with AppleCare had this problem earlier that day. I do have a friend who works at an Apple Retail Store. She hadn't heard of it. I believe it's a small, isolated problem only discovered by widespread means of communication. It should still be taken seriously, I mean did we blow our (Taptic) engines guys?