(Note this was done because the regs governing SynVis [AC for the purists] cite that you can not have two different terrain warning systems running at the same time in an airplane - they must both provide the exact same alerts at the exact same time and since the EPGWS algorithms are proprietary, there was no way we could guarantee there wouldn't be a conflict at some point, hence you only  get one at a time.)

As for the squeal, I don't know what to make of that.   The closest thing I've seen before is that I inadvertently had the volume for the aural alerts set pretty low one day when I was flying low and I heard some unintelligible chatter in my headsets the whole time I was flying out in the countryside at low altitude.  Turns out it was the terrain pull-up calls from SynVis and as soon as I increased the volume bar setting, it all made sense and I shut off FLTA for that flight.   That of course was not a squeal, just some "noise" in my headset.    I can't imagine what an altitude-based squeal is caused by.  We'll have to brainstorm some ideas on Monday.