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."
I also believe it can be a case-by-case scenario. I think this because 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.