What could cause air to be pulled into radiator
 

Have an 02 Camry 4 cylinder. The car retains pressure overnight in the radiator - the morning after driving the release of the radiator cap produces air bubbles in the over flow tank and a noticeable hiss of air from the filler neck. Car is not running hot and no CELs. Fluid is not being pulled into the radiator from reservoir after driving due to high pressure.

Here's what I have checked so far:
Exhaust - no white smoke
Oil - Looks fine no evidence of antifreeze
Replaced radiator cap
Hoses and clamps appear to be good
No fluid leaks found
Compression - all cylinders 145 psi +/- 1 psi
Exhaust gases in radiator - no evidence, fluid remained blue after test
Bleed air from system - was hearing the "waterfall" noise from within dashboard when accelerating. Put car on ramps, took rad cap off and ran for 45 minutes. Saw lots of big air bubbles at first then bubbles stopped. The overnight pressure retention stopped for about 2 weeks / 150 miles - but now has returned.

So it seems clear that air is being pulled into the radiator from somewhere. Beginning to think I have a small head gasket leak that is not showing any other signs.
Before I settle on this conclusion - does anyone know of any other potential source that could cause air to be pulled into the radiator?

Thanks for any info!

There are coolant dyes that will glow under black light to show leaks.

This engine is known for having an issue with weak head bolt threads in the block. The threads can pull out resulting in a head gasket leak and loss of coolant.

Feel the foam insulation on the intake manifold (rear of engine) for moisture.

Do a block test for exhaust gases in the coolant.

Thanks Joey! I have done a block test - fluid did not change colors so no indication of exhaust gases in the radiator. Would this eliminate the head bolt issue you referenced?

Also foam was dry - no evidence of moisture

Only one place the radiator interacts with the engine to produce air pressure . . . the pistons. Somewhere, there is a leak in the head gasket letting compressed air from a piston into the coolant system. Do a cylinder leak down check (with the radiator cap off). I'll bet that one cylinder will produce bubbles.

Check water pump for signs of leakage from weep hole/bearing.

Check upper and lower radiator plastic tank areas. With age the plastic can start to have micro cracks.

How about the radiator cap?

Plus overflow tank and small hose. Coolant is pushed out and pulled into the radiator from the overflow tank.