Hi , Our desktop Pc is making strange sounds e.g. like alarm sound when switches on & we can hear the fan sound but screen stays blank with NO SIGNAL message....

Open the case up and reseat each memory stick (Take it out and put it back in again.)

reseat the Graphics card also.

It could also be a fan fail/failing alarm or overheat alarm if your pc is equipped with this.

Does it make the noise on startup or when its running.

On start up the first responses normally apply whereas mine is if it does it during use.

The alarm sound is probably indicative of overheating issue, if you are happy doing things with the innards of your PC check the fan on the processor is secure.

Clean the CPU fan + heatsink of dust.

Does that fan spin when you power on?

With power off, remove all PCI cards/VGA/RAM and plug back in again. I doubt it is a loose connection TBH as they would normally be indicated by a series of beeps (number/length of beeps tell you what is wrong) rather than an alarm sound.

a 2 tone 'siren' (alarm) sound "up, down, up, down" like the classic police siren sound that kids make is usually a heat issue due to a fan thats failed.