* Crows cackled like a chorus of witches
* Crows cackled like a rusty gate swinging in the wind
* Crows cackled like children laughing hysterically
* Crows cackled like a bubbling cauldron
* Crows cackled like a storm breaking
* Crows cackled like a symphony of broken glass
The best simile for you will depend on the specific effect you want to create.