Simple:
* Creak
* Screech
* Groan
* Rumble
More descriptive:
* A slow, heavy creak
* A sharp, metallic screech
* A deep, groaning moan
* A low, rumbling vibration
* A soft, whispering rustle (if the bed is being dragged on a carpet)
* A rhythmic thudding (if the bed is being moved by bouncing it)
You can also use onomatopoeia to create a more realistic sound effect:
* "Crrrreeeeaaaak..."
* "Screeeeech!
* "Grrrroooooan...
* "Thud... thud... thud..."
Ultimately, the best way to spell out the sound of a bed moving is to use your imagination and choose words that best capture the feeling you want to evoke.