I had the same issue with mine. I had to swap out the sound ROM with a different one that fixed that issue. When I'm home tonight I can tell you what ROM version I had and what version I'm using. But that's what I had to do. I will edit this post later.

Unfortunately it happens rarely (at least it did for me) maybe once in about 10 hours of being in attract mode. It's not only annoying, but it's like someone turned the sound of the game up to 11.

So you need to change the sound chip ROM at S2 on the A/V board. I originally had v1.0 but changed it to v1.1 to eliminate the loud screech.