Crackling on AM Radio when warm

Have a 2009 Fabia Greenline Estate. When I put the radio on AM, it's initially fine. Then suddenly, there is very loud crackling. It seems to be related to temperature - i.e. as something gets warm the crackling starts. I was getting 10-15 mins on AM over the winter. Today it was warmer and I got less than 5 mins. I put the radio on after I done the supermarket shop - engine was off, and the crackling was there straight away.

Any ideas please? I could swap the radio for another - but I'm not certain that will resolve the issue.

Thanks!