To remove noise from a previously recorded vocal track, you will need to use recording software that can take the problem noise in the recording and change it so that the sound is no longer heard. A number of effective programs are available, ranging from free software available online, such as Audacity, to more expensive professional utilities, such as Protools.
Most recording and editing programs allow you to highlight sections of the recording after you have imported the file. Using your cursor, select a section of the vocal recording that only plays the noise that you want to remove. For buzzing and hums, this is typically found at the beginning of the recording, although you can find it at any point when the vocals are not present. When looking at the visualization of the recording, the section containing the noise should resemble a mostly horizontal line.
To change the noise that is present throughout the recording, you will need to make a profile of the sound. Select "Effects" or your program's equivalent. Then select "Noise Reduction" and "Get Profile From Selection." This step tells the program what to look for when removing sound from the full vocal recording. Remember that each program may vary, so consult the help menu of your particular program for assistance with specific commands.
Now that you have isolated the noise you no longer want to hear, highlight the entire vocal recording and select "Noise Reduction" once again. Then click "OK."
Listen to the recording again to ensure it's "clean" and free of the noise you wanted to remove.