The flash is completely invisible to my eye and others that have posted on the subject. I have gotten pics of deer looking at the cam and sniffing it so I don't know if it is invisible to the deer as it is to me or if they can hear something or just know something is foreign.
I tested the cam in a manner similar to what you write when I received it and I could not see the flash. However, there is a very audible filter clunk when it switches from IR to color and vice versa so the animal that creates the first trigger of each change in mode will probably hear the filter.