It could be one of a couple of issues. It is a little hard to explain but does it sound a little harsh like a buzz that changes frequency with engine speed or does it sound more like a cleaner tone that changes frequency with engine speed. The harsher sound would be from ignition noise and the "cleaner" tone would be caused most likely from a failing rectifier in the alternator. The easiest way I can think of to determine which one would be to remove the belt for a moment and start it up and see if the noise it there or not with it running. If the noise is gone then it's the alternator and if it's still there it is an ignition issue. Either way you need to figure it out as your radio is telling you something is wrong.
Also since you mention you just changed the battery I would be inclined to suspect the alternator but I would also make sure all the connections around the battery are good especially make sure the grounds are clean and tight where they attach both at the battery as well as the block and body.
Something may have been loosened up when changing the battery.
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Today's Featured Article - Madison's County - by Anthony West. Philip Madison has been a good friend of mine for quite some time. He has patiently suffered my incessant chit chat on the subject of tractors for longer than I care to remember, and on many occasions he has put himself out, dropped what ever it was he was doing, to come and lend a hand cranking handles, or loading a find onto a trailer. Although he himself has never actually owned or restored a tractor, he was always enthusiastic and always around helping with other peoples projects.
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