Noise During Calls - Sounds Like MOH

I am having a strange problem:

During a SIP or IAX call, both parties hear a very faint “music” in the background. It may just be noise, but it definitely has a “beat” and almost sounds like music-on-hold.

I have tested this with multiple soft and hard phones, and it happens on all of them, which leads me to believe that the PBX is at fault.

Has anyone else ever had this problem, or does anyone have any idea where it might be coming from?