High pitched chitter - where from?

[12boy]

I have found that noise seems to travel on my brompton so that I thought a bottom bracket was failing but it was actually the suspension block creaking You can check that by putting all you weight on one pedal while using your brakes to keep the bike from moving forward. The block is affected, perhaps more by pedalling, than by bumps in the road. In olden days front end creaking on regular bikes was cured by greasing the inside of the stem holding the handlebars. It is an easy fix. If your hub is dry it isn't easy to find new cones, so cleaning and relubing the hub is a very good idea. A dry hub will usually feel gritty when you spin it..

 

[User10571]

So, who wants to know where the chitter was coming from?
Instinct, instinct, instinct.
Always go with it.
Unless you have form for being wrong. :P
It wasn't the main hinge, but it was the mating surfaces of the hinge in the bars / stem.
I applied a little grease to both and the chittering was slightly reduced.
I subsequently applied a MkI section of inner tube, thusly:

and it is totally silent.
Like a nun's fart.
Not yet sure what I am going to do as a long term solution which is maintenance minimum, or preferably, free - but at least it appears that I have identified the source of the chittering.