OK, I did some home testing and came to my own conclusion
The noise on my recording was caused by the 1/8" mini plug rotating just slightly inside my fanny pack. I tested it at home and the only source of noise is when I rotated the mini plug itself, and whether in the Mic In or Line In, it's the same exact problem. I rotated a little and the connection is crippled, then I roted a hair and it's fine, and so on. I must have had the plug in the perfect spot to cause this issue because it's difficult to replicate the cable issue at home.
I concluded there must be a problem in the dual XLR (F) > 1/8" mini cable at the point where the wires are soddered to the 1/8" plug. I bought it from Cascade in July, 2002 and it's seen a good amount of use so I'll just order a new silver cable from someone else. For the record, the cable worked very well for me for five years, so it just needs some repair.
-Tom