Problem with my soundbar

[RobertoRenish]

So i have this strange problem with my speaker that after some use they start making weird sound like exploding popcorns and i can't find the source of the problem or how to fix it, i'm not an expert in electricity or the components and i will appreciate some help.

Problem 1; Distortion when lowering volume: 

 

Problem 2; Distortion while not having anything connected to the speaker: 

 

 

No problem 3 (more like a normal operations); Zero distortion while playing music:

I'm not sure if the house is correctly wired but my guest is that it is, i have never have trouble with any other electronic device in this house and also this problem started small and it became what it is today with a timeframe of 2-3 months. Turn in off and on the speaker fixes the problem but not by much, the more i let the speaker off the longer the problem is fix.
And i'm lowering the volume form my iphone so i dont think the problem is the knob.