That used to happen a lot more back in the 60's and 70's than it does now. I remember hearing the local Top-40 station playing through my Fender Bassman amp in 1966. Usually that is caused by the instrument cord acting as a big antenna, picking up and feeding RF signals into the input of the amp. Sometimes just moving your amp a foot away did the trick, or standing in a different spot. But more often than not getting a well-shielded instrument cord instead of the cheapest one you can find did the trick.
That's actually the main reason we used shielded cords for our instruments. The two-wire lamp cords would transmit the signal from you bass just fine but would also pick up all kinds of hum and some radio stations. If you mistakenly pick up a speaker cord and use that for your instrument cord, you will often get that type of interference because it's just a fancy 2-wire lamp cord with different ends. so make sure you are not using a speaker cord. I'd suggest that you try a few different instrument cords to see if that eliminates the problem. When you find one that fixes that problem, buy a spare one of the same brand, length and type.
Also, be aware that Radio Frequencies (RF signals) are fickle things and atmospheric conditions can drastically affect them. When I was a kid living in Santa Maria along the central coast of California, night time would drastically change what I could pickup and listen to on my little transistor radio. Although I couldn't hear it during the day, I could listen to KAFY in Bakersfield (was top 40 then, now Spanish Language station), KRLA in Los Angeles, and XERB, which had studios in LA but the transmitter was in Mexico. That was so they could transmit with 50,000-watts, which wasn't allowed in the US at the time. It's the station that had Wolfman Jack! During the day, the airwaves were too crowded and I couldn't get any of them. At night they would come and go.
So don't be surprised if this is an intermittent problem and don't go spending a lot of money trying to fix it before you try a number of different instrument cords while you're experiencing the problem.