This has been my experience too. Take some shoe conditioner and use it on the tongue and the surrounding inside areas. It might take 3-4 applications/wearings but eventually it will go away. If you're sure it's not coming from the tongue, I would still try conditioning the leather a bit wherever you think the squeak is coming from.
Thanks for your reply.
However, I'm afraid I can't attach conditioner anywhere to solve the problem because the squeaking seems to come from the sole respectively the bending of the sole when walking.