More than likely the receiver. It might need to serviced or cleaned.
A simple thing (perhaps a temporary fix) is to try WITH THE RECEIVER TURNED OFF, work the volume control back and forth several (10-20 times) times from min to max point.
ROTATE THE RECEIVER VOLUME CONTROL BACK TO MINIMUM, turn on your equipment and play some music at your desired listening volume.
We use to have to do this with with some of the switches in the Navy before we tested them.... It only took us about 10hrs the first time to determine it was corrosion on the contacts that was throwing our readings off.... These were switches that would only be operated in an emergency, but needed to be tested every 6 months.
