I sincerely hope the knocking sound you hear is NOT a deep knock, because many times a deep knock upon starting with an engine that has been sitting overnight, especially on a cold day, could indicate overly worn crankshaft main bearings. If the mains are worn excessively, the oil that normally stays between the crank surface and the bearing will leak out and upon starting, the engine will knock several times until the oil pump pressure builds up. I hope, if it is a deep knock, that your problem is not nearly as drastic and may be engine mounts or even an exhaust pipe against the frame. A sure-fire test for bad main bearings requires a decent oil pressure gauge, meaning one that shows a meaningful range of oil pressure from 1000 TO 2000 rpm. At normal operating temperature and at a steady cruising speed of about 1500 RPM, put a load on the engine without downshifting and observe the oil pressure. As RPM increases, pressure should increase toward its normal upper limit. If it drops, your main bearings are badly worn. Now, remove your foot from the accelerator and observe the oil pressure. The pressure should stay steady or drop. If the oil pressure rises, your main bearings are worn past limits. Unfortunately, I have not yet owned a MB with a oil pressure gauge that would allow for this type of test, but thankfully, I have not yet worn out my main bearings either, so I guess I should not complain, since I have owned a '75 240D, an '85 300SD and currently have an '85 500SEL and a '84 1000SEL AMG.