Ihave a 15,000 gallon inground pool
It was good x awhile and now
Salt level keep staying low
Why?
Iadd 10 bags pf 40 lb each
And 4 more in a dif week and still
Says add...
Does it need more ?
Or is amalfunction??
Ithink my son trow amarble in pool
Which is stick inside filter
Ihear a bubbly sound

Welcome to TFP!

You can't rely on the generator to give you an accurate reading. Take a water sample to a pool store or get some of These strips to test it.

The noise you are hearing is air in the filter. It sounds exactly like marbles rattling. You have a small air leak in the suction side of your system. Use some pool lube on the O-ring on your pump strainer basket lid and make sure it is tight.

Which salt syste do you have? Some systems can indicate low salt, based on the cell condition. If it is old or scaled, it can display low salt, so you're in fact adding salt when not needed. Rather, as John said, have the water tested for salinity to ensure what your salt level is. Then, remove the cell and inspect for scale build up. Acid wash the cell as needed.
If your salt system has a high salt limitation, you will need to be careful with just adding salt when it says so, because if it is due to a scaled cell condition, once cleaned, it will tell you that you have too much salt, and that dilution will be necessary (partial draining and refilling).