its interference from the microwave.
a microwave produces a 2.4GHz radio wave similar to the frequencies of many household devices like cordless phones and you guessed it, bluetooth.
you might notice it with certain things when someone turns on the microwave.

heres a good example of radio interference,
we had a jackaroo at work one time , whenever the bonnet was up, the car running, and you keyed up the uhf, it would turn the car off.