You can actually find the answer in this very forum from a few months back, although as you see it seems to be nothing more complicated than the noise it makes.
When journeys were completed, carriages were hauled away again, by a shunter. Shunters were ubiquitous and there commonness made them somewhat despised by spotters. But things change and as the numbers diminished the good old English Electric shunter (by now officially a class 08) got a nickname. They became gronks.
When starting the engine from cold it makes a distinctive sound that can best be written or described as gronk.
Does it sound gronky here? It does sound unique to me. I like the way it sounds right at the very beginning of this short: https://www.youtube.com/shorts/6s3vUDEEd0Q
