Ok this phrase in your original post has me a bit confused>" if I apply a little pressure at idle, in park to the serp. belt knock can be heard"
Is it a belt noise your refering too? Or is it a metalic noise coming from your engine?
If its a belt noise then just put alittle silicon spray on the belt and or check for a bad idler pulley, but if it's coming from the motor you need to describe it better I.E> light valve train tapping like a lifter or loose rocker arm or is it a lower end knocking noise like a spun bearing.
Please elaborate, more info and a better description of the noise is needed. JMO
Cp