FWIW, no wooooo from my ANC for several years now. It was always intermittent, not showing until a trip to Vermont for Thanksgiving 2013. Then it became persistent, say once a month in winter, now gone?
Here's a thought. The root cause is a chassis/frame/drive train resonance that's been in cars for years, but which no one noticed as long as engine speed is constantly changing. I grew up driving a clutch, so I always listen to an automatic's shift points. My wife's Hyundai hybrid is a 6-speed. Driving a C-Max, the biggest difference initially was the lack of engine speed changes. I'm now comfortable with a car driven by a "stationary engine" designed to run a constant RPM.
Designing loudspeakers, you quickly learn that resonances are a lot easier to hear than drop-outs; peaks stick out more than dips. You don't need much energy to excite them, and once they start buzzing, you can't miss it. I once measured a box with a resonance in the mid 400 Hz range, "middle A". Never heard it, except when using a frequency generator to zero in on the exact frequency!
Ford didn't have the luxury of only suppressing musical notes. They had a broadband source that would continuously change base frequency by a factor of 6, and stay there for a while. The driver can easily run the engine at the exact speed needed to excite any resonance. That's a tough thing to counteract. Ford did a pretty good job in my book.
And the wooooo is now gone because the chassis has aged and I do have one case of very hard braking on rough road that would have loosened things up. If yours still wooooo's and you haven't pulled the mic, maybe try looking for a bumpy road?
Edited by fbov, 16 January 2019 - 04:32 PM.