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.