This does happen occasionally during periods of high-levels of noise on the line. The barely-audible buzz you hear is the sound of the filter network as it deals with incoming noise. Once noise levels are back to normal or below, the filter ceases to make the noise. There is no inherent problem with this, rather it is a sign that the product is doing what it is intended for.
This slight buzzing is usually not apparent unless one puts their ear near the unit during its activity cycle. My unit does the same thing, occasionally.
Hope this helps.