Cunegonde (1.9l WBX manual) has a similar problem. In her case, it's probably the transmission grounding strap - if I tighten those bolts, problem goes away until they loosen again. I use a torque wrench and have replaced the fasteners, so the female threads in the chassis are probably worn. Which gives me an idea of how many times the transmissions and engines (I'm about to install her 4th one!) have been removed and replaced.