My mountain bike makes a grinding sound when riding it?

I have a hardtail mountain bike with hydraulic disc brakes and I'm suspecting that it's because my wheels are out of true. However, the sound only occurs when I'm mounted on it and riding because I've spun the wheels manually using my hand and the grinding noise isn't there. Does anyone have any... show more I have a hardtail mountain bike with hydraulic disc brakes and I'm suspecting that it's because my wheels are out of true. However, the sound only occurs when I'm mounted on it and riding because I've spun the wheels manually using my hand and the grinding noise isn't there. Does anyone have any other thoughts on what the issue might be?
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