Henri,
Great to see them both and they run well. Yes the noise is at least constant, which becomes background after a while. They are both such beatiful classic LGB locos too. I also am really fond of the early undecorated stock. Plain and elegant.
A chum brought a growler 2060 Diesel to an indoor show we did last December. It was good to see that run too, and it ran as just well as yours
If the track they run on has been outside for a while and the line includes points and or crossings, the sound would be a whole lot different. The screach would become a police siren type up and down wail as the power collection from the track was repeatedly lost and regained.
The growler mechs were always pretty good current collectors on curves, but on straights they tend to 'hunt', so in the garden it is mainly the straights, points and crossing that they can have trouble with.
To combat this you ideally need to clean the top inside corners of the straight sections track as well as the top itself. This helps reduce the wail a lot.
Good luck with what you are doing - Next step a permanent track outdoors and maybe a non-growler mech to compare!
James