MK3 is loud during parts of printing  

Re: MK3 is loud during parts of printing

fast gap fills like that tend to make things rattle more than they normally would, but something has got to be loose somewhere.

Re: MK3 is loud during parts of printing

That is defiantly a rattle. Something is loose.

Only advice I can think of is rotate your infill 45 degrees, and figure out if it's the X or Y that's causing the rattle. From there, check EVERYTHING. (I mean, you could just check EVERYTHING... but the less you know about the issue, the harder it is to find... and it could be either X or Y... which is 80% of the printer.

On a side note.
My Mk3 made some TERRIBLE sounds on the Z hop at times when printing. I'm guessing there was a burr or something on my screw and it made a TERRIBLE sound. However the more I print, the less it does it. Only time I notice it, is at a spot around 120mm up... which I'm guessing is about 2-3mm high. (so likely a single little burr) as it's gone after about 20-30 layers. It used to do it REALLY bad (so bad it woke me up in the middle of the night). When I first got the printer, it made terrible scratchy sounds EVERYWHERE. Bearings were SUPER loud, and the right side Z screw was rather grindy. I thought I might need to file a warranty complaint, but I ruled the noises as a "Likely go away after a breakin period" and it did.
(Your sound is something loose. It will get worse, not better)

EDIT: It sounds hollow and echoey, which makes me think Y axis. Also, There was a fill spot the X axis was going wild, and the sound wasn't as bad, as when the Y axis went crazy and the X axis didn't move as much... I would look at your Y axis, for anything loose, rocking, or flopping. I can't "visualize" that sound, so I can't help beyond a suspicion of the Y axis.