I had a problem that sounds identical to your description, I like your description, I'm not too good at verbs or English as you will see, because, prior to today, I described it as plastic farts. The bikes I've had in the past that had gaiters on the forks, had small tubes for air to escape when the forks were compressed. One bike had gaiters that didn't have that feature. They were clamped to the forks with plastic zip ties which did the job, but after a while they loosened up. Still good enough for small bumps, the gaiters just expanded, but on bigger bumps the air escaped against the resistance of the zip ties.
The way I discovered the problem was with the front brakes applied to the stationary bike and pushing down on the handlebar. I solved the problem by poking a small hole at the rear of the gaiters.
You could get lucky having a helper with good ears, this might help to pinpoint the location of the noise. Probably not good to ask a helper who looks something like this:
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