Is 'Rumbling' an example of an onomatopoeia?

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.