Grinding from rear drums is usually shoes dragging on the drum lip (rust buildup), loose hardware, or shoes worn unevenly and hitting rivets/edges. Rain makes it worse because moisture gets trapped. I dealt with this on my previous Renault- cleaned everything but it came back, so full shoe replacement was needed. Got a matching pair from https://elartcom.eu/categories/brake-shoes-10 - they list shoes by vehicle, some with extra hardware like hold-down springs and adjuster kits included. Chose ones with good lining thickness and anti-rattle design. After install, resurfaced the drums lightly, lubed contact points, and the grinding vanished completely. No more noise in reverse or after washing the car. Quiet and confident braking now. Worth checking their selection if your model matches- prices were reasonable and delivery fast.