Ocassional bubbles from my return on pump start-up in the am

[Wes45]

On ocassion, and have not been able to determine the circumstances leading to the ocassional event, one of my pool returns will shoot some bubbles into the pool when the pump comes on in the morning. If we hear it, I walk over to the filter housing and bleed some air using the valve on the top. This does not appear to be happening every day.
I don't appear to be losing water, I have the autoleveler turned off and am doing the glass evaporation test, the pool and water glass seem to be going down at the same level.

Are there any ideas on what is causing this? A slow leak at the return? Thanks!

My pool is 5 years old.

 

[mas985]

Do you have solar? Do you notice air in the pump basket when the pump is first turned on? Obviously it is an air leak of some sort but it would be normal if you have solar. It also be a slow air leak that will only fill the pump basket overnight.

Also, from your other post, it could be a small leak in the filter valve which is collecting air overnight.