Hi,
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If you keep the file 16-bit, it will be processed in 32-bit in the project. If you convert it before, you don’t get more data from the file anyway. So you can keep it in 16-bit.
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First os all, don’t use Noise Gate, please. This will Mute the very low (meaning in Volume) signal. But once the real signal comes, the noise will be back. And it will be much more noticible, because the human brain reacts on the changes. You can try any noise-reduction plug-in, but be very carefull with it, please. In 99% I just end up with the result to keep it, as it was, with the noise, as the best tested solution.