Singlespeed & Fixed Gear - shimano freewheel probs

I just picked up and installed a brand new shimano freewheel (16t) onto a Suzue hub and I'm having a problem with it. When I back pedal the bike by hand (with the bike upside down) the freewheel resists at certain parts of the rotation, and it makes a quiet grinding sound. The chainline is almost perfect. One thing that kind of concerned me is that theres only enough threading on the hub to fill about half of the threads in the freewheel, but I dismissed it since the suzue hub and shimano freewheel seems to be a common combo. When riding, it seems completely fine though. I guess my question is for those of you that have these freewheels. Do you experience the same thing with your shimano freewheel? or is completely smooth on the coast? could it be defective? thanks.

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is your chain too tight?

I didnt think that was possible. Should I let some slack out?

I didnt think that was possible. Should I let some slack out?

chainrings aren't quite round, nor are cogs/freewheels, so if you get it tight to the loosest spot in the rotation the tightest in the rotation is bound to bind. take a bit of tension off and notice how it will be slacker at some points than others.

Take the chain off. If you just turn the freewheel by hand, does it still grind?

Thanks. That really helped. The drivetrain is much smoother now. The freewheel still seems a bit gritty/rough but I'll deal with it. Thanks for the help!