Strange Laptop Speaker(??) Noise Interference ?

[slickcondo]

On my laptop, I have 2 USB slots. On each slot, I connect a 4 way USB platform to it (I think this is the word for it - it let me have the ability to connect 4 USB items to each of these platforms).

My USB powered modem is connected to one of these platform outlet and it has been working without any problem for years until a few days ago. I then started to get interference sound from the speaker (like from a old vinyl record that is dirty while it is being played - if you are from that era when these are popular !). This only happens when I am watching a video from YouTube or other online files, but not when I play a video that is already downloaded in my latop before this problem started.

1) I then connect the modem to the other platform thinking that the first one may be causing the interference, but still no improvements.
2) Next, I changed the modem, but it still give the same problem.
3) I have no addirional new hardware connected that may cause this interference.
4) Finally, I connect it directly to one of the laptop USB slots and this seems to solve the problem.

Does anyone know why connecting to a USB platform is causing this interference and not when connected directly to the laptop USB slot ? Doing it this way meant I can only have one 4 way platform connected.

I am using Windows XP Pro with SP3 and IE7.

Many thanks in advance for your help and comments.