Grinding noise coming from rear end

I was cleaning my bizango just a moment ago and when I was pedalling I heard a grinding sound at the rear end, put my ear where the noise and it seems that the rear disk break is grinding on the pad. Is that normal or the rear hydraulic break is too tight?
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As for the chain, you use a good chain lube, I like squirt water based wax as it doesn't make everything all messy with gunk.