I have a 1990 300e with 275,000 miles. Don't laugh. Great condition daily driver that I keep up the maintenance on. Have what I at first thought was an exhaust leak at front of engine but now I can tell is coming from the area of the air/smog pump. Its not a bearing sound more like an exhaust leak. And this sound only lasts for maybe a minute or two (I think that time period is consistent with the operation of the air pump). I found that there is a check valve and shutoff valve in the air pump system. Maybe they are clogged? In addition the car has had some hesitation on acceleration, coughing and poor idle. Any thoughts would be appreciated.