Try switching the heads between two channels to see if it is the head. If it turns out to be the head, they come apart pretty easy. The grinding sound makes me think that would be the culprit, but if it isn't, I have 3 of those dosers and have to replace the tubing inside the head every few months as it cracks with time. You can end up with a pretty big mess when this happens, so careful where you put the doser. When the line cracks, it ends up not pumping anything into the tank, so this might be the problem. I posted tear apart picture steps to the head in another thread several months ago, but it's pretty easy.
Dan
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