Is the word 'Rumbling' considered to be an example of onomatopoeia? I know that the word 'rumble' is an onomatopoeia, but I am not sure if that applies to 'rumbling' as well.
I think that rumbling is an onomatopoeia hence the example of "thunder rumbling across the open sky"
However, I believe it can also be a case-by-case scenario as sometimes you may be describing the rumbling. This could be used like "the booming rumble of thunder echoed in the valley"
You should also be aware that adding -ing usually does not change an onomatopoeia. For example using the onomatopoeia crackle, vs using crackling has no difference.
So, yes I believe that rumbling is an example of an onomatopoeia.
I was unaware of evidence that 'rumble' is an onomatopoeia but happy to take your word for it. I have less evidence than that for it but 'rumbling' would simply be what someone is doing when they rumble. The fact that -ing does not spoil the onomatopoeia of one's rumble is a happy accident. If you want the rumble stopped it's up to you.