Strange noise through speakers

[Rick Cabell]

Helping a neighbor with a 06 997 that had a dead battery. Took it out and charged it on low for almost 2 days. Fully charged battery back it car. It started right up. Right away I was getting a strange sound coming from the speakers. The attached video shows the radio on, but it doesn’t matter. Even when you shut the car off the noise continues for about a minute. For now I am going to leave the negitive cable off until I figure it out.
Turn your volume up, you can hear it clearly.
Does not want to load video. It sounds like blown speakers, but they are not cuz you turn radio on and it sounds fine, just the extra noise.
Anyone ever heard anything like that.