What causes Valley Oak trees to make noise?

I am trying to find out if anyone has ever heard of Valley Oak trees making noises – not from insects or animals or birds or frogs living in/on the tree, but the tree itself.

A little background: I live on a fairly steep hillside and built my house and deck around mature Valley Oak trees (about 40–50’ tall). My upper deck is built so that if you stand up, you can see over the top of the trees (although a few trees are slightly higher and I can see thru the top 1’-2’ feet of the Oaks).

Standing on my deck today after finishing photographing the hundreds of hummingbirds I feed every day (yes, hundreds), I heard the tree around me making noise – at first I thought it was a sprinkler or dripping water that I could just barely hear in the distance. But after a quick check, it was clear the sound was literally coming from the Oak tree branches surrounding my head. (I hang my feeders in the tree canopy to offer shade for the hummers as they feed). Normally, I’d never have been able to hear the sound as there is usually a breeze that ruffles the leaves -even the slightest breeze will cause the leaves to rustle. But today was perfectly still and I was mystified/amazed to hear the trees making noise that I’ve never heard before despite living in this house for 15 years and spending a lot of time on the deck. However, I must admit, I’d never stood still long enough to notice the sound before.

Today was special as I was taking a short close up video of the hummers as they feed at “rush hour” – ie at dusk. So I was standing still so as not to disturb the hummers who were only 2–3’ away. When the light began to fail, the hummers left for the night and that’s when I noticed the tree making the noise.

The sound is so gentle that I could only have noticed it on the quietest of days (no traffic noise, no crickets, no frogs, no wind).

One theory is that the noise is some how related to temperature as the sun sets and that the leaves are contracting as it cools and thus popping ever so slightly. But it seems a strained explanation as the temperature drop is gentle and gradual and not something I’d expect.

As I’ve only just noticed the noise, it may not be present all year round. So perhaps the sound may instead be coming from the nearly mature acorns. Could they heat up during the day and then gradually contract, causing the noise?

Mystified in California would like an explanation or be pointed to someone who I could talk to who is expert on such things (if there is an expert for something like this).