"a soft thud" is a correct and usable phrase in written English.
It can be used to describe a light, muffled sound made when something falls or hits a surface. This phrase is often used to create a sense of atmosphere or to add detail to a description. Example: The book fell from the shelf with a soft thud, causing Lily to jump in surprise.
Somewhere, a piece of fruit dropped with a soft thud.
The sounds came in quick succession: a soft thud, a dropped ball — bounce, bounce, bounce-bounce — and then the child began to wail.
She heard a soft thud and a rattle of glass; Neil heard it, too, from inside the room and turned his head.
When she spotted him outside, she immediately yanked out her earbuds, pushed back her chair, and hurled herself against the picture window, landing with a soft thud.
"It struck the left front wheel with a soft thud," Henning Mankell wrote in "Faceless Killers," the 1991 novel that introduced Wallander, a police detective in the windswept precincts of southernmost Sweden.
Sruly Weinstein unleashed one of his wily underhand pitches, a soft thud issued from Mr. Dachs's bat, and a dispirited sigh rose up from the Lobos' dugout as ball met glove, bringing the inning to an end.
After taking care of some paperwork, he went down to the gym, changed into running clothes, and jogged across Memorial Bridge, along the Jefferson Memorial, until he heard a deep, soft thud.
They observed three types of ice losses, each with a distinctive and detailed sound signature: the splash of an ice block falling off into the water; the crack of a fragment sliding down the glacier's rough surface; and the soft thud of an underwater ice chunk breaking away and floating up, followed by a secondary impact as it surfaces.
Every so often, a soft, deep thud caught the ear, sometimes followed by a snigger.
Her feet made soft thuds when she landed.
At the time, I wondered about the soft thuds I had heard the night before at the pub where I was having a drink.
