Best way to track this down is to locate which of the inputs into the mixer is sourcing the noise.
There are 10 different sources on that board for sounds. All ten of them can be seen on the left side of resistors R73 through R77.
Best method to find culprit would be to compare the output tone to the outputs at each of these ten sources and see which matches. This is quite easily done with an O'Scope. Might be able to do this with your logic probe. This will narrow it down to which section of the board to look at next.