I'll assume you are familiar with the buzzing sound that any modern cell phone or nextel unit can introduce into an audio circuit and that THAT isn't what you are hearing. That can be introduced into a microphone audio circuit during transmission from a field subscriber unit. I have also heard it being introduced on outbound transmissions, presumably from a dispatcher's personal cell.
Now, if what you are hearing is truly "in the background" as you say, then from the description of "a tray of broken glass" could it be tire chains while a mobile unit is in motion? I must say I've never heard anything on the air like what you said, but lastnight when I heard an officer going as fast as he safely could to a call, I did hear a similar sound and I am certain what I heard was chains.