I've had my 2016 EcoBoost for about a month now. Just recently its started to make a noise that sounds like a rock being rattled around in a tin can, but today while getting on the interstate the speedometer stuttered a good bit while this rattling was happening. The confusing thing to me is that it only happens under heavy acceleration (ie getting on the interstate) and I can't replicate it any other way. It does have an aftermarket exhaust that is loose on the right side (it vibrates a decent bit when at idle at stop lights), however it has been loose since I bought it and this rattle has only recently started. I have no clue what this could be and would really like some insight.