The thought now is that the noise is from the catalytic converters going bad. On the 1975 450 SL, the catalytic converters are in the engine, mounted to the exhaust manifold. From this forum, I learned how to test them so that is the next step. Diagnosis! Wow! What a concept! If anyone has some insight on these catalytic converters, please post! They were only made like this for two years so there is not a lot of info about them. Thanks for the birthday wish! My car was supposed to be ready in time to celebrate but no such luck. I will be celebrating late this year.