The AM radio I just got to work again did the same thing for a few minutes. Fiddled with the volume control and the tone control and started getting some actual sound. This was not mounted in a car so how the antenna was orientated also helped a lot.
After the crackling lessened I started hearing actual radio static, then I knew it was actually receiving something and it was making it to the speaker ... then a bit of fiddling with the antenna and it brought in a few stations.
Try giving the volume and tone pots a workout and see if it gets any better.
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