Working on a Black Knight. While in attract mode there is a repeating "thud" sound playing over and over again. It plays when the displays are blanked in attract mode. When playing a game the audio is mostly correct (thud goes away), but some sounds are wrong and I think it's based on when those sounds are played (to align with when the thuds would be played, timing-wise). In the video, the machine is not getting tilted, it played the tilt sound when the ball went over the shooter lane switch, and again during game play for when it hit a different target. It's not actually tilting.

Video of issue:
https://www.dropbox.com/s/ngazfoc3o3td7x6/VID_20210824_092009438.mp4?dl=0

I disconnected the control lines for the audio board to isolate it from the MPU/Driver board and the thud goes away. Comes right back when the control lines are re-connected. I pulled out my logic probe to check the lines and when the displays blank pins 3, 2, and 7 (on the sound board; sol 9, 10, and 13) dip. I went ahead and replaced the PIA (IC36) for the sound lines, didn't change anything.

The game plays fine other-wise. There are no issue with the switch matrix or lamp matrix, so I don't think it'd be anything on the main address and data lines.

What do you guys think the issue would be?

I ended up swapping the MPU and driver boards from a different machine. The issue stayed in BK. Swapped the main sound board and the issue went away. So it's something on the sound board. I'll dig into it later... ... probably when I need to repair the machine I took the boards out of. lol

I'll update this thread with the solution when I find it.

Thanks for the pointers.