Macbook Pro 15" hum/noise from speaker

Hello guys, I bought my MBP around 3 weeks ago, everything is fine except the sound output, when my speaker is connected to the lap top it hums during and after(around 30 seconds after the sound then the hum will goes away) any sound come out from the lap top, let's say I am playing music, in background i have the hum, when the music finish there is hum around 30 seconds then no more hum. Even if there is small sound plays in the laptop it's with hum (ex. i receive new email which cause small beep or buzz to notify me there is new email then this hum also starts.)
It's really annoying, I really don't know what to do. I disconnected the charger but no changes at all. This thing only happens with external speaker, with internal speakers and headphones there is no hum (My speaker is fine there is no hum with my other laptop).

I appreciate if anyone can help me.

Welcome to Mac Forums.
Even though you say that you have tried your speaker in your other laptop, have you tried other speaker(s) in your MBP?
The problem could be with the speaker, yet alone that it's one speaker, and not a set of speakers!

6string said:

Welcome to Mac Forums.
Even though you say that you have tried your speaker in your other laptop, have you tried other speaker(s) in your MBP?
The problem could be with the speaker, yet alone that it's one speaker, and not a set of speakers!

Click to expand...

Finally it solved, thanks dude you were right.
the problem was the cable of my speaker which goes to output jack, I must change the cable, because now I tried with my friend's cable no more hum..