3 Cadmium

Re: Beep

At this moment in time the system runs normally with no unexpected shutdowns or BSOD's (at least you haven't mentioned otherwise). The hardware and memory diagnostics are all clear.

The 2 beep error disappeared when you re-seated the RAM. But you're left with this single beep and click.

The beep could indicate faulty / failing hardware but on the other hand the system could run for another 10 years. I'd certainly consider backing up my personal / important data just in case though.

I'd open the system and re-seat HDD.  While the system is open I'd take the opportunity to give the CPU fan and heatsink a good clean. I'd also replace the CMOS battery.

I'm not going to suggest fitting new RAM or new HDD, or even a new mainboard as this could prove unnecessary and a waste of money. Without definite error codes and unstable behaviour it's too soon to call.

The only thing you can do at the moment is monitor the system.