In February 1357, some residents of Rennes heard underground sounds which made them understand that Lancaster had ordered the digging of a tunnel under the ramparts, hoping to make troops quietly emerge into the heart of the city.

Informed, Penhoët ordered the residents of houses near the city walls to hang in their homes copper basins containing metal balls so as to determine the exact location of the gallery thanks to the vibrations caused by the mining work.

An even more famous episode in this siege involved a herd of pigs (2000 to 4000 according to Michel de Mauny) which Lancaster, knowing the state of famine within the walls, made to feed in front of the Mordelais gates in order to lure the people of Rennes out of the town[b] The captain of Penhoët again distinguished himself by having a sow suspended to a postern of the gate; its calls attracted the pigs, which ran into the town before the English could react.

[9][10][11] A little later, another reprovisioning was brought by Bertrand Du Guesclin, who entered the town with carts full of rations, distracting the attention of the English duke by this trick, making him think that this was the arrival of a troop of German mercenaries.

The history of Brittany by Henri Poisson and Jean-Pierre le Mat states that the siege was lifted following a treaty between France and England, but indicates neither the date, nor the amount taken by the Duke from the residents.