I suspect multipath dropout, a temporary interruption of the radio signal. The audible effect may range from a slight “swishing” noise to a complete loss of sound. Such dropouts may be experienced even at relatively short distances by a mechanism called multipath interference. Part of the signal from the transmitter (which radiates in all directions) travels directly to the receiver, but some of the signal is reflected to the receiver by metal objects or other structures. When the “paths” of the direct signal and of the reflected signal(s) are sufficiently different, they will interfere with each other when they combine at the receiver antenna. If the interference is great enough, partial or complete cancellation of the signal occurs, resulting in a dropout.
