It may be the starter or it may not, but you would be well advised to treat it like it was because it sounds like that car may leave you stranded somewhere. I was always lucky with most of my cars that the starter would give me some warning like yours is. It makes it much easier to get it to the shop or parts store. I have changed out starters with less symptoms than that and have never been disapointed. If you are a DIY you may try to change it yourself. I am fairly good at fixing cars and am no way afriad of try most things. If your car is a 84 300d your starter is pain in the @#$ to remove and replace. I tried for three hours with no luck and paid someone to do it. Total cost is about $300 parts and labor. I hear that you want to make sure that it is a bosch rebuild.
diesels are hard cars to start. It is important to have your starting systems in good order.