My cat is shaking when he lies down?

I have noticed a few times that my Maine Coon cross Ragdoll is shaking when he lies down, mainly at the beginning of an evening when I get home from work, can anyone help? He has done it 3 or 4 times today when laid out on his side. I thought it was purring before now but when I get close he stops shaking. He is asleep now and not shaking and really don't know what to do for the best. A trip to the vets may be in order but why does he only do it when laid out by me, is he pleased to see me as the shakes don't last long? He doesn't like to be petted much but likes to follow you around and sit close by you. Many thanks in advance for any help you could offer. Leah