This Corona Four portable typewriter types quite nicely, but is horrendously noisy. Of course the platen is the main source, but the typebars return-drop was also very noisy and all little bits help.
On these Corona machines however the typebar-rest was not of felt, but consists of a rubber tube held in a channel-strip. This tube will surely have been soft and dampening when new; today it is rock solid. The net effect is a nice, loud 'clack' as the typebar drops back - makes sense as a hard hollow tube being struck on the side is pretty much the design of a tam-tam / jungle-drum.
To replace the typebar-rest, the first step is to remove the front panel. The four small screws were corroded fast on this machine, but leaving a small (small!) drop of oil on each screw for a few months solved that completely. Now also easier to clean and polish the bit behind the top bank of keys.
A small rubber-band will keep the typebars out of the way and allow the typebar-rest to be taken off easily, by removing the two small screws at either end.
The old, hard rubber can then be pried out of the channel-segment. (Taking care not to bend the fairly flimsy metal part.) Before you do this, you may want to measure the height of the rubber in the channel.
Not having soft rubber tube at hand in the right diameter, decided to replace with felt. To make a strip of the right thickness and height, had to fold-over a thick felt with a third layer inside - spots of glue to keep it all in place. It needed clamping tight to let the glue dry and keep it in shape - does look a bit like an exotic caterpillar...
Not to worry about the length, to be cut to size after fitting, but do fuss about the correct height. The height needs to be large enough to provide enough damping, but if it's a bit too high then the typebars are too close together at rest. Then the type-slugs will foul each other and/or make extra noise as they drop back. (As I found out... : )
With the channel-segment fitted with a new felt strip that is tucked in nicely (sharp screwdriver to push it to the bottom at all sides), it can be fitted back onto the machine. All the typebars should now again line up nicely - and make just a little less noise as they drop back.
And then of course mount the front panel again (the typebars just peeking over the rim). A slightly less noisy, but still very loud little typewriter.