Depending on atmospheric conditions (including winds, relative humidity, air temperature), flight altitude, ground elevation and background noise, it is possible to hear aircraft noise at cruise altitudes, e.g. above 30,000 feet. This noise manifests itself as a near continuous rumble if you are in a very quiet location where the only noise is background noise and natural soundscape.  We have a set of noise measurements to demonstrate that this is the case. Here we show examples of simulations with FLIGHT-X done on an Airbus A330-343 (Rolls-Royce Trent 700 engines), flying at M = 0.80 at a cruise altitude of 33,000 feet. A summary of results is shown in the table below.

Atmospheric Conditions    EPNL    SEL    LAeqT    PNLTM   Lamax   Lmax   (all data in dB)

ISA, RH = 20%                       63.5   63.6      43.3        42.2        42.2      53.6

ISA, RH = 50%                       63.7   62.0      41.8        58.3        46.2      56.0

ISA, RH = 80%                       67.7   65.5      45.2        60.7       48.0       57.2

ISA +10 C, RH = 50%           66.1   63.1      42.8       57.6        44.2       54.8

Thus, noise heard on the ground increases with air humidity (all other parameters being the same). At a fixed level of humidity, noise on the ground increases with air temperature. Note that there are several simplifications here, including the critical assumption that the atmosphere has a uniform humidity level - certainly difficult to prove.