I doubt they are afraid of the sound of rain on a metal roof. I have two donkeys who would much rather stand out in the rain that in their stalls (which they have free choice to go in and out of). I wouldn't worry about them - donkeys are smart and will go in when they want to.

I can't imagine there is any way to eliminate the sound of rain on a metal roof. I have one and I am so used to it I don't notice it any more.

I thought you would find this photo amusing. I have another of one of my donkeys standing out in driving, sideways rain! This photo was taken after an really long rainy spell. This donkey likes to put things in his mouth!!

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I read somewhere that the spray on truck bed liner will greatly reduce the rain sound on metal roofs. You can buy it by the gal from Home Depot or Menard's. It can be sprayed-brushed or rolled on. It's about $50 or so per gallon but it goes a long way. Comes in several colors. I've used it on the interior of the chicken coop and cat kennel. Makes cleaning a breeze, also cats don't claw up everything except their claw logs. An added plus is mice and rats can't chew through it.

We have 7 miniature donkeys that do the same thing! We have a nice shed that they could stand in, but they choose to stand in the rain. I don´t think it´s because of the noise on the roof, though.