When you are recording vocals are you using monitoring? Sounds like a feedback loop, or you're recording in some kind of echo chamber

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If you're monitoring, try using headphones and turn your monitors down... see if it mysteriously 'goes away'... if it does you've probably either got something hooked up wrong, or bleeding or whatever.
Other than that... maybe check to see if you have an echo effect turned on somewhere that you are routing the signal through and not realizing it.