Build a PC High pitched whistle coming somewhere from PC

Now, I know this question has been asked quite a few times, but the results always vary and it appears really dependent on the hw. So:

Recently, I have been hearing an interrupted high pitched whistle from the case. It is not too loud, but interrupts when I need to ex. read something and concentrate. It is not a singular long beep, but series of small beeps.

When I opened the case, I tried to listen where it goes from; it does not appear to whistle from the electric source; and CPU / Graphics / Net card / Motherboard are too close to each other to properly determine. I tried to listen with a straw, but it always appears to emanate from someplace else when using this method.

I tried to dust everything, firm the screws, nothing helped. The issue started about two or three weeks back. I purchased the PC about a year ago and the user have not reported anything similar; the last change was adding a new CPU fan about four months ago.

The Rig:

Mobo: ASUS M4N78-AM SLV1004
CPU: AMD Phenom(tm) II X6 1055T Processor
CPU Fan – Coolermaster Hyper 212+ Evo
Ram – 2x2 GB PC26400 AM1
Vga – AMD Radeon 7700
FDD: 16 in 1 apacer sln4123
HDD seagate 250 GB sln E78C
Cdrom – DVDRW LG sln 5393

Advice very much appreciated!