Scottfish

When the brushes get badly worn the heater fan begins to slow down a lot. It can make all kinds of obscene noises, and it may only blow when jarred around a bit.
The brushes are soft metal and they are spring loaded to be pushed into the armature. When they wear they get shorter and shorter until they no longer reach. That is why the jarring can sometimes make them work.
Also the contact failure will cause the fan to blow with less force. The brush replacement should be a DIY job. I fouled up the first attempt by not getting the brush wires soldered on there correctly. You will need to pull the fan motor and it is not hard to pull apart. Just bend the tabs and it should come apart. Then at least on mine the brushes were mounted on the end plate assembly.