Would a pillow sachet be a gentle way to handle this?

I’d check the ordinary things alongside any ritual. A phone vibrating, pipes, a loose bed part, or something shifting near the pillow could make a small knocking sound. Try moving the bed or pillow, and keep a brief note of when it happens. If the repeated waking is affecting her, a healthcare professional can help look at sleep-related causes too.

A sachet can be a comforting symbol without making any claim about what is causing the knocks. Rosemary and clove are options if she likes them, but I’d put a closed pouch on a nearby table rather than under her pillow. That avoids loose herbs, strong scents, or possible irritation. She can set an intention for quiet and rest without asking anyone to leave or disturbing the ashes.

Keep it simple and let your mother choose what feels reassuring. She could calmly state a boundary before bed, such as wanting the room to be quiet while she sleeps, then use a non-smoke cleansing or a small sachet as a personal reminder. No need to decide who or what is behind it first.