This greatly reduces the noise from the extruder motor. The wooden backplane of the Ultimaker did act like a drum.
To find out how much noise this upgrade can remove, carefully un-hook the feeder while printing, and hold it in your hand. Answer: it makes a huge difference.
Update1: separate, much better holder for the bowden coupling, and other tweaks.
Update2: upper hook should withstand PLA creep better.
Print the "feeder_suspension" STL twice, the "middlepiece" and the "coupling holder" once.
I sliced with Cura, with two walls, infill 20% and 30% both worked fine. I think natural (clear) PLA has the best spring properties, but I have printed the second version in gray PLA, and it works fine too. I had to turn the bearing holes through sandpaper because there was a printer blob inside.
Note: the new material feeder will tilt slightly to the side, and follow the movements of the printer head. It's probably a good thing, because it relieves some tension that was previously in the bowden cable. But it sometimes adds a new clicking noise, when the grip on the wooden back plate shifts during printing. Also, the filament seems to make slight rattling sounds when it is not feeding straight now.
Known problems: PLA under mechanical load will creep over time, even at room temperature. Check and re-tighten the motor screws after the first few weeks. The upper hooks were a weak point, I had them slide out after a few months (impossible with a fresh print), so watch out. The latest version should be better, but only time will tell for sure. You can fix the upper hooks with a zip tie or packing cord through the wooden back holes to prevent it.
The design was done in Blender using 2D splines. You could probably extract them from the .blend file and laser-cut it. It was based on the laser-cut drawings by Ultimaker. There isn't much left of the original, so I dropped the silly non-commercial license restriction. Please consider my design public domain. Attribution is appreciated but not required.