Electrical interference from case fan causes speaker hiss

Hi,

I tried this question in Audio/Visual, but I guess that's not where the fan experts hang out.
I just hooked up my onboard sound (Asus A7N8X-X - Realtek ALC650) to a NAD amplifier and noticed a rather alarming steady background hiss when playing CDs. Poking around inside the case I discovered quite by chance that unplugging the case fan stopped the hiss completely. The fan is nothing special - an Enermax ball-bearing fan, ref UC-8FAB, with manual speed adjustment. The fan was plugged into the motherboard header. I'm wondering if electrical interference from case fans is a known problem? I've come across a small number of references to apparent interference with floppy disk drives and parallel ports. If I buy a different fan, will I likely get the same problem?

Please help.

I didn't find the right solution from the Internet.

References:-
https://arstechnica.com/civis/viewtopic.php?f=7&t=620400
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Thanks!

Hi Joe,

Thanks for posting your question. 

It seems you have isolated the hiss problem to the computer fan circuit. By replacing the fan with same kind, or placing the fan with another brand, the hiss problem could still persist. 

I cann't rule out partially defective mother board, that is the fan circuit on the mother board who is to blame for the hiss problems.