Occasionally, we get a loud buzzing sound in the calls recorded by mixmonitor – it completely obscures the voices, although the people speaking do not hear the sound, so it is only in the recording.
Has anyone else experienced this, and did you find the cause?
Try giving us a clue. How did you install the system, what is the OS, are you virtualized, what trunks do you have the problem on, If they are dahdi, what is the hardware. maybe you have a hair-dryer near you machine 
The OS is Ubuntu 12.04 X64 Server. Asterisk 1.18.20.1, FreePBX 2.10, real hardware. Compiled and loaded from first principles. The conversation between the call agent and the remote client was clear (no interference). Th recording contains a buzzing noise which obliterates the voices - sometime the buzzing starts and ends during the call, i.e. the start and end of the recording are clear. Thousands of calls are clear with no buzzing - just the occasional call. I don’t think it’s wireless interference - I think it’s a software or load related problem.
I have had this problem at two clients, one using a SIP trunk and dahdi-dummy timing source, and the other with a Sangoma A102D 2-port ISDN/PRI card providing clocking.
I have a similar problem on a system running on real hardware, Asterisk 1.8/FreePBX 2.10 on DAHDI trunks using cheap OpenVOX A400 card with MG2 echo cancellation. Conversations are perfect but recordings occasionally are very poor audio quality. Recording is not a critical function so I have not spent much time trying to fix it, but I can confirm that it is possible to have good audio during a conversation and poor audio for the recording.