Air in Coolant System

I have started to get the water sloshing sound coming from under my dash, and based on my research on the forums that indicates air in the coolant system getting trapped in the heater core. I have done a coolant block test (combustion leak test) and it did NOT indicate that I had exhaust being blown into my coolant system from a blown head gasket.

If it's not a bad head gasket, what would be some other possible causes of air getting in my coolant system? I am going to bleed the air out as suggested, but want to find the cause of the issue.

 

Bad radiator cap?

 

Does your heat still work good?
An external coolant leak will also allow air into the system.

 

Do you still use A/C at this time of the year? Could also be condensate water trapped in the evaporator box.

 

If he uses the defroster he uses the AC.
Good though

 

vacuum fill

pressure test

 

I had "sloshing" noises under the dash, no leaks but the air moving sounds was noticeable. The coolant recovery system was full but not working and I was low on solution in the radiator when I removed the radiator cap. The cap was original, ten years old and over 150,000 miles. I could see no leaks even when letting it sit, idling after it was at operating temperature for forty miles.
For the heck of it, I bought a new cap ($33 at Toyota, $13 at NAPA). After a week now, it seemed that was what I needed. The noise went away, the recovery tank was working fine, and I didn't need to add coolant. It was sucked back into the radiator.
Just one guy's completely anecdotal experience.