hi forumers....

i was wondering if anyone could help me as i'm not sure if this is health related issue or its just my cat's behaviour...

it doesnt happen all the time but when it does he will vibrate n will last till he wakes up... sometime there will be pauses between vibrations, sometime it'll just goes on & on.... it doesnt happen when he sleeps on his tummy or curled into a ball... n doesnt vibrate when he's awake or half-awake, only when he's in deep sleep...

could somebody explain what happening pls? it is normal?