Now that... Is a new one on me.. The MiFi uses two different frequencies well removed from television channels. I have Never had any interaction between either my Cell Phone (Which is also a WI-Fi Hot spot) or a Verizon Jet Pack I used for a month. and my televions, and when I was using the Jet Pack TV reception was, to say the least, very subject to interference (Minimum usable signal strength, no signal at all if I did not have a full blown Batwing, complete with wingman and sensar pro, like 1000 times better than a Jack).
BUT.. possible it is..
ONE thing I do notice......
Cell phone handshakes with the towers from time to time (normal operation) and if the phone is too close to an AUDIO device (Example, if I'm using AUX-IN on my dash radio to play a Mini-Disc) the cables, which are not always that well shielded, pick up the phone's handshake.
It may be your coax cable is low quality or a connector is loose allowing this to happen.. Many companies use only RG-59 and lowest bidder at that epically for the antenna lines.. QUALITY RG-6 gives better shielding (you want double if not quad shield) and stronger signals (less loss) at ALL frequencies.
But replacing the existing cables... NOT an easy job.