white smoke?

every now and then after running for a while if i slow down when approaching island's traffic lights or anywhere i need to stop or slow white smoke appears to comefrom the exhaust, if i stop the smoke comes from under the bonnet, when i pull away it sometimes stops straight the way other times after maybe a mile. It doesnt do it every journey, just now and then if i have been doing short trips and then go on a long trip but not all the tlme, it has 160,000 on the clock and uses no oil. I have an M.O.T. advisory stating a slight split in the exhaust flexi hose ? not sure... this is a 1.9 tdc vauxall vectra diesel estate 2006 any idea's please ... phil