If @CPU 's advice of flashing the BIOS doesn’t help (seeing as the mobo was replaced, I’m not 100% certain that’ll help), and you’ve checked that your RAM is not only working but also in the right slot(s) in your motherboard, I agree that the peripherals or the dirty power are what I’d look at next.
SEMI-RELATED STORY TIME INTERLUDE!
I recently started playing Rocksmith again, which if you don’t know, is a game where you plug a real guitar into your PC (or console) via a jack -> USB cable and play along (kinda like Guitar Hero, but more advanced). I noticed this annoying buzzing sound, which also registered on the screen as input so I know I wasn’t imagining things, which I figured out after a while stopped if I touched one of the strings. Or, as it turned out, any metal bit on the guitar.
Here’s what I’m fairly sure is happening; the wall socket my PC is plugged into isn’t grounded. The noise in the current running through the PSU, into my motherboard, then the USB cable and then my guitar was big enough to actually be audible unless I touch a metal bit of the guitar and act as the grounding myself.
In short; your PSU can’t filter out all the noise of the incoming power.
How this related to your story; I’d indeed try to disconnect the peripherals, see what happens. In particular the two things that are connected to power aside from your PC: speakers (usually) and your monitor. If you expect dirty power, you may have a reason to, and as I just explained the resulting noise may reach further than expected. Who knows, maybe it’s interfering with each other.
To be perfectly honest, I have 0 reason to believe this will have any effect… but after what happened with my guitar, it wouldn’t surprise me either.