Noise from mobo coming thru speakers

I have a new PC connected to my existing speakers, viaaudio jack at rear (on mobo), and regularly hear gentle hissing. I hear noisy static, and an intermittent whine that changes pitch depending on what's going on in the PC.
This did not happen with previous PC
How do I prevent this?

 

This can happen when components aren't adequately shielded, or emit too much RF noise. Unfortunately, there's nothing you can do about it.