Is 'Rumbling' an example of an onomatopoeia?

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.