Air bubbles from floor drains

[Woodinsac]

I have a one year old in-ground pool. The builder went out of business shortly after our pool was finished (Aqua Pool and Spa). I have noticed that when the filter is on high speed with the cleaner running, two of the four floor drains have air bubbles that rise to the surface. I have released the pressure at the filter and there does not appear to be any built up pressure in the system. What could be causing these and what is the fix that I am looking at? Or is there nothing to worry about?