Yes, thud is a Scrabble word!
...and is worth 8 points in Scrabble, and 8 points in Words with Friends
thud
Verb
1. Make a dull sound
2. Strike with a dull sound
3. Make a noise typical of an engine lacking lubricants
Noun
1. A heavy dull sound (as made by impact of heavy objects)
What Scrabble words can I make with the letters in thud?
3 Letter Words
2 Letter Words