Here's why:
* Short vowels are typically followed by a single consonant. In "rumble," the vowel "u" is followed by two consonants ("mb").
* Long vowels can be spelled with a vowel digraph (two vowels together), a silent 'e' at the end of the word, or a single vowel followed by a consonant and then another vowel. In "rumble", the "u" has the long vowel sound because it is followed by a consonant blend.
Therefore, "rumble" has a long u sound.