Sorry did not read the text where you said you had disconnected the switch headers.
So you are not getting any POST. No beeps, even with the RAM and GFX out.
Keep the CPU in.
Make sure the CPU fan is plugged in and working
Make sure you have plugged in the CPU 4 pin power connector (if one required)
Now power up that. It should give fault code beeps.
If not do you have an onboard speaker? If not make sure the case speaker is plugged in to the correct header.
Best if you state the model of motherboard. Can then check if it's supposed to beep if no CPU installed.