Hi Yal
Well, I have E220 1993
From the way u described the starter I tend to believe it might be the problem cause this descripes the sound I hear.
You now the normal sound was just to hear the sound of the engine as u pull the string, but it fades then engine starts. It reminds me with very old cars that used to be run by rolling a shaft Or old planes when they used to run then by spinning their fans

I don't know if you agree with me, but after I did the jump start for the guy, I didn't face a problem!! I just faced the problem after 5 days! I'm not an expert but i'm trying to think in logical way!!