I recently ran into an issue with some old filament. When printing, I noticed that there were bubbles appearing in the extruded filament, along with a popping sound. After some research, I found out that the issue might be due to moisture in the filament. Both PLA and ABS filament have a tendency to absorb moisture from the air, and since I am located in lovely Florida, the 100% humidity gives the filament plenty of opportunity to have water in it.
During the printing process, the filament is heated above the boiling point of water, so any trapped moisture will turn to steam and expand. This results in bubbles in the filament.
There are ways to deal with it, as I talked about in this post, but the best way is to store your filament in a dry environment so it cannot absorb moisture to begin with.
I used a Ziplock Weathertight storage box (I picked mine up from a local Office Depot), which has a foam seal around the inside to help prevent airflow when closed. I also purchased an Eva-dry rechargable dehumidifier to constantly try to dry out the air inside the box (every time you open the box you’ll have fresh, humid air enter the box).
Since I started storing my filament in this way, I haven’t had any issues with moisture in my filament. Some people also like to modify the storage box to also be a filament spool holder, where the filament can be fed directly from the box. I have not done that with mine, but you may want to consider it yourself!