No. An onomatopoeia is a word whose sound relates to its meaning; shush is an example. But since you can't say the word still without moving your mouth (unless you're a ventriloquist) and you can't...
No, neither of those words is an onomatopoeia.
Babbling Brook.
An example of a sentence for an onamatopoeia is : "The pipes went clunk and clinck at the old house across the road."
Boom! Pow! Smack! Bzz!