High pitch noise on booting

I just got the board, loaded the SD card and plug the HDMI cable and the power. The green led was blinking still, but the board started to produce a high pitch noise. The screen remained blank.
It is difficult to describe, but at the beginning there are some variation in the noise, and after a few seconds, it stabilizes into the same tone. If I gently press the HDMI cable down, the noise will stop; but if I remove it completely, it remains at the same level.
I assume this is not expected behaviour. What can I do?
I have double checked the PSU and it is according to specifications and positive polarity in.
It is difficult to describe, but at the beginning there are some variation in the noise, and after a few seconds, it stabilizes into the same tone. If I gently press the HDMI cable down, the noise will stop; but if I remove it completely, it remains at the same level.
I assume this is not expected behaviour. What can I do?
I have double checked the PSU and it is according to specifications and positive polarity in.