As far as I've seen, Sierra gears are a re-box job just like "Yukon" gears from Randy's Ring and Pinion. There might be crap gears in there if you paid the low end price, and there might be good gears in there if you paid something more than their rock bottom price, same deal as with Randy's. In reality you need to ask the guy to go check who really manufactured the gears you're ordering in the box they're sending you, don't let them just tell you "Sierra gears".

I believe the "Sierra" gears I've installed have come from Drivetrain Wherehouse here in Cali. If it's not Drivetrain Wherehouse, then it was Drivetrain Direct.

Of course none of this helps your gear whine problem, but good luck pulling it apart to see what the problem is. And remember, dial indicators can be had fairly cheaply ($30 will suffice), as can gear marking compound, so there isn't really any excuse for not using these two vital tools to ensure a good setup.