Hdmi Receiver 2.1 Any Disadvantages Of Connecting Blu-ray To Surround Sound Receiver By Optical Rather Than HDMI?

Any disadvantages of connecting Blu-ray to surround sound receiver by Optical rather than HDMI? - hdmi receiver 2.1

I bought a Blu-ray (Sony SDPS350) with my HDTV (Toshiba 37XV505D go) and 2.1-surround sound system (Onkyo HTX22HD - if the sub / amp / receiver are one unit). I currently have the Sony connected to the Onkyo HDMI and the Onkyo to the Toshiba connected back to HDMI. For some reason, the Onkyo sub / amp / receiver hums "when HDMI to Sony, which is linked very annoying, but not when connected via an optical audio cable.

So my question is twofold:

1. Can someone explain why Onkyo sub / amp / receiver "buzz" at Sony are running (the rumor is not available when the game with Sky +) and what could be a solution?

2. Was the only way (I actually cure or hide) the problem is that Toshiba Sony) via HDMI (video only connection and then connect to the OPTICAL Onkyo Sony audio cable. I understand that this means there will not be able to get true HD sound from Sony, but I have a 2.1 system, I'm not sure that I receive such benefits from any road. Are there any other downsides this solution? Sound quality is significantly reduced?

Your help is appreciated!