The grub screw is just a Blank for the grease nipple.

I have fitted loads of SBC rails/bearings and they are ok. They do have one little thing that can happen and I'm pretty sure this is what your problems is.?

The bearings have thin steel plates that helps retains the bearings and stops them falling out when off the rail. If not careful when fitting then these can get damaged, often it's because the ends of the rails have not been ground smooth or have sharp edges or just rough handling.
The bearings a have 4 ball runs 2 at top 2 at bottom the Top two runs have thin plate each side and the bottom 2 share a wide plate that sits in the bottom and rides just above the top surface of the rail.
It's this plate that gets damaged and when the balls roll over it makes that sound.

They are easy to strip and clean but do over a container has all the balls will fallout.

To strip remove the 2 tiny screws and the Grease nipple (you can leave the grub screw in) that hold the wiper seal covers, behind these you will find 2 Allen key bolts.
These hold the end caps onto the main bearing block. These end caps hold the thin steel covers in place and all the balls so when you remove all the balls will fall out so make sure your working over a container.
When you pull the end covers off they will have 4 tubes sticking out each cap, these hold the balls and form the circuit. If you carefull pull at them and hold the end cap you'll see they detach from the end cap.

Now you will be able to examine the thin steel plates for damage and clean everything. If you look carefully at the ends of thin steel plates and the inside of the end caps you'll see how the register to each other with slots in the end cap to hold in place.

To reassemble then put one end cap back on full.
Then put the tubes back in the other side but leave end cap off.
Next put the thin plates back into the registers on the fixed end cap.
Now while holding the thin plates roughly in place load the tubes with balls. For the wide thin plate I suggest you lightly grease the back side to help hold.
When tubes are fully loaded you may have a few spare balls, these go into the end caps held in place by grease.
Now put the end cap back on beaing careful to locate with thin plates and not drop a ball.!!! .. . . . Do all this over the conatiner.
Thats the bearing done.

When putting back on rail turn the bearing upside down so you can see the balls and watch that none get pushed out the race by the rail. They shouldn't but if your thin plates are damaged or worn then it can happen.!

Thats it and your back to together.

Will post a short vid when it's loaded up.