Bass Guitar / Amp … vibrating string Loud?

I just bought a new Line 6 LowDown bass watts 150 amps for my Hohner B Bass Guitar … And every time i turn the master voulme up 3 / 4 of the way it really hard and this vibration becomes louder and louder every second that I my Figner Channel A, and when I put my finger on the A chain, he stops suddenly, when I removed it begins .. someone knows why he made this noise and how can I solve this problem?

How close you are to the amplifier? You are probably generate feedback loop between the pick up the guitar and amp – as when a system sound of screaming when you get a microphone too close to a monitor or a loudspeaker. Stay away from the amp, try to play in another place in the room because the room may have a resonance that enhances the tone of the string A. Finally, you may have to turn the amplifier down a bit if one of these things do not work.

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