Created an account just to respond to this because after months of new cables and different devices trying to figure out what was causing this issue I figured out how to solve it.
In Windows 7 go to Recording Devices by either clicking on the sound icon at the far right on the task bar or typing "Sound" into the Start Menu search bar. Navigate to the Recording Devices tab and right-click on the "Line In" option and choose properties or double click it. Change the tab to "Enhancements" and uncheck "Disable all Sound Effects" if its checked, then check "DC Offset Cancellation," then click Apply at the bottom.
This is all I had to do and its impact was immediate. I hope this works for others and that it works for other operating systems as well, as I only have Windows 7 to base it off of. I made sure to leave a comment here because this was the first result when I searched on Google and despite it being so old it didn't solve my problem! My hope is that others in the future will come across my answer in the pursuit of solving this issue as I did in my search.