need some starter advice

[79ON40s]

I have a 79 k10, and i have had problems with the starter since i bought it, I was using those cheap Orielys starters, and could never get them shimmed right, so I bought a Delco, I bolted it on, and it worked awsome, no shims, no noise, no problem. after a while though it seems to get louder, and then i get a really loud metalic noise, like the bendix is trying to kill the flywheel, it seems that if i pull the starter off and and clean the surface, bolt it back on, that the noise seems to go away for a while. any help would be great, thanks