Answer: No, it is not.
Step-by-step explanation:
Firstly, the meaning of an onomatopoeia is a word created to sound like a sound when read aloud.
An example of this would be "pow" or "sssssizzle."
The word "silent" does not describe the lack of auditory perceptance in a specific environment. Instead, an onomatopoeia to describe the sound of silence would be... well... nothing. LOL
Really, the only valid form of onomatopoeia for the sound silence would be sound effects that go from being silent to being louder. This method is used by some japanese "onomatopoeia producers."
There is probably some debate on whether you can use certain words to describe silence and consider it an onomatopoeia. Some people may argue that "static" or "hush" aren't just adjectives and can function as an otomatopoeia, but I definitely don't think so.
"My words like silent raindrops fell" is a figurative statement, and I've never really seen an onomatopoeia used in a metaphorical sentence.
I hope this helps :)