Problem with video card ASUS ATI Radeon HD 5670 (Elite 7500 series MT)

Hello,

my product Nr. is B5G74EA3#BCM, with new power supply (Seasonic Energy Knight SS-500ET).

My PC had no HDMI output, so I've installed video card ASUS ATI Radeon HD 5670 (about 4 years-old).

After its installation, when I switch PC on, this POST front power LED and audible code probably occures (according to manual): "Red Power LED flashes six times, Beeps stop after fifth iteration", indicating pre-video graphics problem (?).

Then PC go on working normally, with no visible problems or unexpectedbehavior.

Any idea what should I do to solve this problem?

I tried to remove the card and to install it again, but with the same result as above.

Or should I try to use another video card?

Thanks