79 pickup starter, gear reduction or direct drive
 

So I just replaced the starter on my 79 pickup, 20R. Wondering if anyone knows the difference between a direct drive and gear reduction starter. I bought a gear reduction and although it starts right up it grinds slightly for a second after startup. I'm thinking this has something to do with gear-reduction vs direct drive even though they all fit the model and year. If anyone has input please help!