Water softener introducing air into pipes

I installed a new water softner 4 years ago and it has been working perfectly. Two automatic regererating cycles go, I noticed that the following morning, when I flushed the toilet closest to the softner, air was introduced into the pipes. This has happened 2 cycles in a row. I also noticed residual water in the salt pellet tank after recycling the last time.
What is my problem & what is the resolution?
Thanks for your help.
Joe

Too much water in the salt tank and air in the water usually is caused by air suction leaks in the brine line compression fittings on the brine line at the control valve, a float controlled safety brine valve or the brine pickup tube.