BTW, I think C18 should be 0.01uF [10 nF] and not 100nF, which gives a low-pass frequency at approx
318 hz. C18 = 10nF will move it up to 3180 hz.
I calculated that wrong before. I used only 5K in the computation. R equivalent should be more like
R = (5K || (5K+5K)) = 3.3K, so
F3db = 1/(2pi3.3K100nF) = 477 hz
F3db = 1/(2pi3.3K10nF) = 4770 hz