any time! and if you have an exhaust leak, you should hear a ticking noise and a 1/3 can of sea foam in a vacuum line would definitely tell you if you had one or not. and the dip stick for the
tranny is right up againt the fire wall around where your air intake is. if your exhaust is fine and your
tranny fluid is ok (your supposed to change it every 30,000 miles but i dont know the last time yours was changed) i would recommend putting your car in neutral, turning off the engine, jacking up the back half of your car then spinning the tires. if you start to hear a strange noise, its time to replace your bearings (bad bearing can cause vibration as well)