while running my E3 I noticed a rather mild but persistent popping noise coming from the Z Axis when homing and starting a print. I took the top off to get a better look (open the door and look up, there are two Phillips screws and the entire top comes off as one piece) and found it’s a single z motor and a belt that connects the two z rods. I tightened this belt and all noise went away. As per usual the belt loose made a thud when strummed, after tightening it Twangs and you know it’s tight enough. To tighten mine i loosened the two bolts that hold the tensioner assembly, try that first, if it doesn’t tighten enough you’ll need to adjust more belt through the tensioner.