I have had this problem with NASCAR ais. However only in certain areas. I have sailed the coast of Morocco
twice and on both occasions it would sound incessantly despite rebooting and no other vessel in sight or on the screen
. It happened for a few hours mid atlantic also. My only suspicion might be inland radar
or some such thing on the Moroccan coast, possibly submarine mid atlantic though doubt whether they would be announcing their presence! Which reminds me I really must ring up NASA and ask for their views. But I will follow this thread with interest too.