Need help, "pop" problem

[Imakida]

Hi there, thank for taking the time to read me.

So, I have a problem, one of my surround speaker (the left one) is making a "pop" everytime a signal come in, like if I change the channel on TV, or start a blu-ray movie. The sound is not too loud, but it's annoying. I know the speaker is not the problem; I tried to invert it with my other surround speaker, but the sound is always coming from the left. My receiver is an Harman Kardon AVR 247. What could be the problem?

I thought it's either in the wire or in the receiver? What do you think? Any help or insight would be greatly appreciated.