I'm still convinced it's engine related for the following reason -
When the noise appears (very abruptly, as mentioned!), after 3-5 seconds it translates into normal engine noise until the aircraft is out of hearing range and fades away naturally.
It 'sounds' like the usual noise of an approaching aircraft (at circa 5,000 - 10,000 feet) has been massively delayed (just silence) due to conditions, wind or otherwise, then it hits you all at once for a few seconds before returning to normal.