Vibration

Deep vibration from rear of car when driving around 50-70kph, does not do it all the time only occasionally, does not affect driving in anyway, more of a nuisance than anything, rev counter does dance around slightly but not to much, have just done repairs to radiator and instead a oil cooler as water got into transmission and transmission oil got into radiator. I was just wondering if anyone has any ideas what the vibration could be?