The values of R21 and R23 are 1M. That is going to make the output a very high impedance. A high impedance is very susceptible to picking up rubbish. To avoid this make the impedances 3K3 or lower. Note that is not make the resistors that value but the overall impedances.
What are you trying to do here make a 16 bit PWM output from two 8 bit ones? If so that is not going to work anyway unless you have super precise resistors which I suspect you haven't.