This is happening to me.
I was recording, as separate inputs, a shotgun mic into input 1 and a wireless lav into input 2. A number of times I was hearing that swishy noise on input 2. After some tests, I found that when input 1 peaked, or near peaked (it didn’t have to peak, just a strong signal), input 2 then experienced that swishy distortion. I called Canon and they said there’s nothing that can be done. I’ve never had this issue on other cameras before.