|I agree with OtheHill - you should have stated right from the start that you were having problems & what the symptoms are. All this talk about RAM has been a waste of time.|
You need to post your complete system specs in as much detail as possible. Since you say it's a custom system, we'll need the make/model of motherboard, make/model of CPU, amount & type of RAM, make/model of video card (if you have an actual card), make/model/capacity of hard drive, & make/model/wattage of the power supply.
Also, explain what you have done to troubleshoot so far. And I don't mean internet searchs. I mean what have you physically done - remove RAM, check connections, blow the dust out the heatsinks, verify that the monitor works, etc. When does this single beep occur? There should be one single long beep at startup, that's normal. It means that the System has passed the POST (Power On Self Test) & that all is well.