Chasing an alignment problem only on 6 and 7 cog
 

8 speed cassette. I've adjusted the RD (Shimano Tiagara). Everything is right except the 6 and 7 cogs. As I turn the pedals at "medium" effort, all seems well. If I turn the pedals fast and shift either from bigger cogs or smaller cogs, I get definite noise when shifting into these two gears. If I keep going, it'll get quiet again going to 5 or 8 and then all the way back to the #1 largest cog. If I am sitting on 6 or 7 and turn the pedals slow, every once in a while the RD will twitch.

I've tried barrel adjustments and can't seem to get rid of it. Any advice?

check you hanger ,it might be bent . also check the chain for a tight link .

If you run in 6&7 most of the time the cassette may be worn in those gears. You may also have a bent tooth on one of those cogs. Just a guess.

I don't know which way you're counting, but if you have greater noise on the larger sprockets there are a few possibilities to check.

The first and most likely is that the RD is a bit high and there's interaction between the upper pulley and sprocket. Shift into the suspect area and check that there's at least 1/2" of free chain or daylight between the pulley and the sprockets. If not use the B screw to push the RD down and see if things improve.

It could also be a hanger issue, which usually shows more critically on the largest sprockets because the pulley is closer.

It's a good guess, but to add to that guess if the cogs are worn then likely the chain is worn so that should also be replaced.

But always do cheap things first like FBinNY mentioned in the post above me, those type of things don't cost a dime so do that stuff first.