I've been trying to fix this for a few days now, and have googled it to death.
My Brompton M6R (2 speed derailleur, 3 speed hub) has an intermittent high-pitched squeaking sound when I'm pedaling. The same pitch as... a mouse squeaking...
I've already dismantled and greased the derailleur idlers and tensioner. I've greased everywhere I can think of, and I've oiled the chain a couple of times.
The sound only happens when I'm pedaling, not when freewheeling. And I've noticed it always happens at the same pedal position.
I've been unable to get the sound when I have the bike upside down turning the pedals.
Should I look at cleaning and oiling the chain properly? - Or could it be the 3-speed hub the sound is coming from? (Although, since it's always the same pedal position, I strongly think it's a rusty link on the chain)