I nearly had a heart attack when my 2014 435i started making this low knocking sound from the bottom end, and the first shop I took it to said it needed a full engine replacement without even dropping the oil pan. A fellow car enthusiast told me to stop panicking and take it to a place that actually knows these N55 engines, and I'm so glad I listened because the guys there diagnosed a failing belt tensioner that was slapping the timing cover, not internal damage at all. They turned out to be genuine BMW Repair Center Experts Abu Dhabi who saved me thousands in unnecessary work, and they even showed me the old tensioner and explained how the noise travelled through the block. If your BMW is making scary sounds, don't just believe the first quote you get, get a second opinion from people who really understand these motors. Now my car is quiet again and I can sleep at night.