Warning: SLF query re engine sync
The past couple of rides on AA TATL 777's I've observed periods of very noticeable low frequency vibration that I'd guess would be the beat frequency between the two engines. Sometimes it's constant for minutes, other times varies in amplitude with a short period (a few seconds), other times it's not noticeable at all. At its strongest it can be felt easily through well-padded seats.
I suppose it's been happening for years and I've just noticed, but given the search for "shareholder value" at many corporations, I wonder if tolerance on some maintenance step has been increased. (Still well within limits, of course.) And of course the underlying question would be what all that energy does for the airframe longevity.
pj