I am using Windows 7. I use the USB 3 to link to the turntable. when I first start recording everything seems fine. But then at about 3/4 of the way through the distortion starts. Sometimes it goes away by itself, but the recording is still dreck.
I don’t know that I ever heard of this happening with USB audio before, but it’s possible. Windows sound channels are not “flat.” They’re aggressively managed for proper voices in conferencing and business communications. You should find all those settings and turn them off.
That’s not the full list, but that’s a good start.
And sometimes it can be down to a bad USB connection - cable/plug/socket.
Unplug and replug the USB device
Try a different USNB socket on your PC
Try a diffent USB cable.
DON’T route via a USB hub
You will need to stop and restart Audacity after re-plugging/re-cabling or use Transport > Rescan Audio Devices