Chevy & GMC Vans

I have a express 3500 van thathas gone through 3 starters in 7 months. I mostly buy on line with all the info and install starter. It cranks and starts fine at first,then a couple months i hear the starter wheel enguage then i hear a whistle noise and i am pretty sure the wheel is not enguaged anymore and it wont start. So i buy a new starter and same amount of time passes and same thing. What am i doing wrong

So when you take the starters out, what do they look like? Is the bendix all chewed up? Do you bench test them? Which brand do you buy, and new or rebuilt?

Either the starters are junk, or it's not shimmed properly, or you have some damage on your flywheel.