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What's Wrong? Electric Bass Feedback

I own a Ibanez Ergodyne EDA 900. When ever I touch the 3rd string (D string) Tuning knob, bridge, any metal touching it, I get feedback sounds to my amp. What's wrong with it. It's not the amp because I tested it with my friends bass. *** I Still don't know what happed but after i took it apart and checked all the conections (nothing wrong) I blasted all the dust out of it and IT WORKED FINE.

Bad ground ?