6 month old kitten constantly sucks on blankets?

I aquired two male kittens after they were abandoned at my work. This was back in February, they had been dumped in our dumpster inside a box that had been duct taped shut! The vet said at the time that they were approx 3 weeks old, I bottle fed them and weaned them onto regular food, now they are ENORMOUS, fat, happy obnoxious boys! One of them has recently started up with an unsusual habit. He curls up on my couch on top of this fleece blanket, but as he is going to sleep he sucks on it. It is really strange. He does not slurp on any other blanket, and seems to be well adjusted, aside from being the clingiest cat on earth most of the time! Why would a kitty do this? I have never seen it before. It is the oddest thing, he will be sleeping away, and you can hear him slurping on the blanket!