Beeps are typically indicative of a failing part in the system, often ram.
I'd suggest doing an SMC reset and an NVRAM reset on the system.
SMC you do by unplugging all cables save for the Magsafe cable, shutting the system down. Press (left side of keyboard) shift, option, command, and power. Wait 5 seconds, then hit power.
To reset NVRAM, on bootup hold command, option, p, r. You'll hear the POST sound as the system boots up, the screen will go black, and you'll hear it POST again. I typically reset the NVRAM 3 or 4 times before I let the system boot all the way up.
If the symptom persists after doing the SMC and NVRAM reset, make sure you've run all software updates. If it persists after that, I suggest you take the system to your local Apple Store or Apple Authorized Service Provider. As always, backing up your data is critical, so make sure you do that ASAP.