Except for internal issues, which is another story, there are some things that can cause it and which in principle can be solved without spending money at first:

Groundloop: To prevent ground loops, all signal grounds need to go to one common point, so connect devices in serie, not parallel or looped.

Non grounded connection: This is a worse scenario, since the only way would be to create a grounding yourself or correct the connections by making it grounded.
The first option would be costless, the second one is absolutelly not, therefore I am not explaining the second one (yet).
I have grounded my equipment (I got a lack of good electrical system in my very old house) by connecting the ground of my power socket to my heating system, which is grounded physically. Just a copper wire in between. Not the most pretty solution, but well it works.

Interference:
1. The speakers might get some interference of your computersystem or other electrical devices. Make sure the speakers are put further away from the interfering source. (testing through moving the speakers, when the humming seems to change, find their best position)
2. Cabling picks up interference. On a laptop, you won't have balanced connections (those nice interference eliminating cables, shielded and working through the common mode signal rejection principle), so all that you can do is to prevent any electrical device or wiring to be close to your speaker cables.

I'll be back...