I've seen this before. I think it's large bubbles of air caught in the turbulance in the filter and pump that gradually disperse. Sometimes just turning the pump off and waiting a few minutes helps, or turning the pump off and bleeding the filter.
zea3 touched on this but for a in ground pool where the filter is above the water line, when you open the bleed valve with the pump OFF, air will get sucked into the filter. The weight of the water pulls in the air. So when the pump is turned back on, the air will be purged out of the filter through the returns. To bleed air from the filter through the bleed valve, the pump must be ON.
Is the pump below the water line then? I have had below the water line filters that have bled better with the pump off. I've similarly primed pumps by turning them on then off again when pockets of air have been trapped and simply running the pump was insufficient.
The OPs pool is an in-ground and most of the time the equipment is above the water line so air will be sucked in when the valve is opened with the pump off. However, for above grounds and a few in-grounds where the water line is above the filter, you can purge the air with the pump off.
If you keep getting air into the filter when the pump is shut OFF (listen for the air bubbles in the filter), then it could be a gasket leak in the backwash valve that only lets in air but does not let out water when the pump is running.
I commonly hear what sounds like air in the filter. I've been concerned with my gasket because one time (yes I seem to like to make the mistake first and then learn) I moved the valve when the pump was on. Any other way to verify this may be faulty?
The pump is probably sucking in air and passing it through to the filter although this should be fairly constant and not diminish over time.
Do you have all of the valves open for the skimmers and main drain?
A quick fix for the oversized pump problem would be to swap out the impeller for a smaller one. I replaced my impeller for a 1 HP Northstar down to a 1/2 HP version and cut my power consumption nearly in half.
All valves are fully open to both skimmer and the main drain. The noise is consistent. Air bubbles at the returns is minimal. What I was commenting towards at the start of this thread was a much higher rate of bubbles in the return flow. That has since diminished since the pump has been running. The constant flow of very light bubbles is still there. I would swap out my impeller but I'm still a bit concerned that I will lose too much flow to provide enough pressure to my mist cooler and to my spa jet returns when being used.
They actually ran the second pump for the waterfall feature. The misters do have to overcome a good bit of head pressure as they rise out of the water about 1 foot before the water comes out in a mist. I actually have to block off two returns to get adequate flow out of the misters.
I've thought that may be the solution but I'm concerned I would just always be running it on high anyways (at least in the summer when I use the mist coolers which can last nearly 6 months in Houston). There's a few different scenarios for myself where the requirements for flow would be different
1. Most common: Pump running in pool mode with with spa spillover, misters in place, some flow returning to spa to circulate.
2. Pump running in pool mode with spa spillover, no misters, some flow returning to spa to circulate
3. Pump running in spa mode, all water to spa
4. In either of the first two cases my pool cleaner could be running as well. I'm not sure this matters as it runs off the booster pump.
I'm kind of stuck between having a pump that's over sized for my piping (1.5 inch) which seems to be causing a very turbulent environment in my pump strainer basket and not to mention the cost, or the other scenario of purchasing a smaller pump and possibly running in to not having enough head to properly operate the misters (which I love because it's so Dang hot here anyways lol)