I have had the same problem you just pointed out here. The cure for me was a Audio Transformer I bought from a Local Grocery Store! Yes, I found a real nice ground loop isolator at a grocery store..it is a model-ES-034 made by SCOSCHE..It was made for use in an automobile stereo unit, real nice unit too, it has two transformers inclosed in a nice plastic case and rca connectors, female and male on both transformers for a stereo. So it works wonderful on a sound card, or a mini disk player. ( you can isolate both in and out)
Now I know your going to say that a ground loop is not the problem, that the 7mhz rf is getting directly into the mini disk, I thought the same thing....but after I tried this little $13.00 dollar unit, I was supprized that all my 40 meter rfi was gone in both my mini-disk, and my sound card.
Not sure if this helps at all, but thought it was worth sending...
73 my friend