buzzing sound on portable xm stereo

Hi Everyone,

I recently renewed my xm subscription. I play music in my car by using a portable boom box. Its called the SubX2 and I attach a Stratus 6 receiver on it and an XM car antenna I put on the car roof.

However, I keep hearing buzzing noises that disrupt the music. Can anyone diagnosed this problem and reccomend a fix??

Fyi- I drive a limo and the company installed an aftermarket GPS unit on the dash board which a coworker said was interfering with my signal. My boombox works fine outside my car.

Thoughts? Thank you!!