open the case, a wire might have found it's way into a fan of sorts that turns itself off after the bootup from standby finished
Typically when I'm not using my computer, I put it into sleep mode. Recently when I start my computer, it tends to make this loud buzzing noise. I think it may be the hard drive. When that happens, I just put it on sleep mode and wake it up until that stops happening. I'm worried that this could be a problem. Does anyone know what's going on?
if what merciless said isn't true, then it's most likely your hdd dying
Is it high-pitched? If yes, then it most likely is the power supply.
If no, then is it consistent? If yes, then it most likely is a fan (most likely the CPU, but it could be the power supply or other fans).
If no to both questions, it could very well be your HDD or your CD/DVD drive, although they sound more like a rattle than a buzz.