No, sometimes it's the armature not engaging properly on the starter due to a burn spot on it. When the brushes are on the burned spot, it does not kick out to engage the flywheel and then just spins, making that noise. One way to find out is to hit the starter with a hammer when it does it--usually that will kick it such that it will then engage. It takes alot to knock the teeth off of the flywheel.
They can be rebuilt, you know. It's a pretty cheap job actually. I always had my diesel starters rebuilt as the equipment dealers wanted a fortune for a new one. Every time some dolt ran a unit out of fuel, the priming process seemed to destroy the starters as they are always (seemingly) right below the injector unit which must be bled of all air--smelly damn fuel everywhere. Always worse when it's hot too.
It can be the solenoid (Bendix equivalent) sticking. It just does not properly engage properly. And, it can also stick when it should disengage. Then you might get a high pitch noise while driving. Had both problems over years with 300D/350SL.
By the time you get starter out (not an easy job), it is probably worth having it rebuilt or replaced. I think someone here was able to get solenoid out without removing motor but I wouldn't want to try!
I found a rebuilt Bosch starter on eBay for under $100. Had shop replace it. With replacement, they didn't have to wait the day or two it takes to get old one rebuilt.
PS: While out on both cars the ring gear was inspected. Slight wear, but no damage. They probably deliberately make it tougher than the starter pinion.
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