Spitting in 3yr old with DS

My 3 year old son has DS and is a "spitter".  He had stopped for a while, but now it has started again and become worse.  He gives "raspberries" at the most inappropriate times (usually right when you are face to face with him).  But, it doesn't appear to be related to his disapproval at the moment, it just is...  Any idea how I can encourage some behaviour modification.  Time outs work on "big" offenses like bitting and throwing, but this doesn't seem like a real time out type of problem...