It’s quite a common thing on Bromptons, there’s another thread on here discussing it.
I had the problem myself, but it’s gone now, I’m not sure if the noise went away of it’s own accord, or if it was because I poured some oil into the hub, either way it doesn’t make the noise now. BTW, it didn’t make the noise when it was new, I’d had it a few months before it started.
Another best check with your dealer first before doing anything.