It sounds like the starter drive gear (aka bendix) slowing down to a stop...

Do you have time when you get no starter engagement and have to make a second attempt.. If so then the gear is failing..
The gear itself can be replaced..

__________________

79 240D my current toy
42 years a Diesel addict

240D sold
250SE sold
220D sold
280C sold
280S sold
300D (2) sold
300CD sold
300DT sold
300SD sold
380SL sold