The constant beep is likely due to memory problem.  Take out all the memory chips and inspect/report their spec.  Then put only one back in DIMM1 to test.
so i got a 780 that was used but when i turned it on its says the diagnostic lights say 3 4 and it beeps like this beeep beeep beeep beeep beeep
Optiplex 780 diagnostic Lights say 3 4 and beeps