Powered USB hub issue

[Sonic Blue Meanie]

I'm chasing an issue with a high pitched noise I get when PT starts up with my mbox2. I've read the issue might be solved by using a powered usb hub instead uf the usb connections on my imac.

I bought a 4-port Kensington hub and connected the mbox2 to it and from there to the imac. Now Protools appears to open but will not play, neither will itunes. When I hook it back the way I had it, still get the noise, but all works fine.

When I go back and reconnect the Kensington, I get the following error message-
"PT cannot communicate with the mbox2. Please make sure the mbox2 is connected and not set to AC-3 mode. I checked prefferences and AC-3 is not checked and it says the mbox2 is connected. I've searched the issue but not finding anything."

and...

"PT could not set sample rate to specified value"

Any ideas?