The TDMA burst rate of GSM frames is 4.615 msecs. There are eight slots within this frame each being 0.577 msec.
The most common interference is caused by the frame burst rate at a frequency of 1/4.615 msec or 216.7 Hz with the 1/0.577 sec. bursts causing 1733 Hz buzzing with 216.7 Hz rep rate.
You sometimes can hear the intermittant 'buzz, buzz, -- buzz, buzz, buzz' when TV or radio newscaster's have their Blackberry too nearby. The strong GSM transmit bursts from the phone is getting into their microphone circuits.