Tapping/Knocking noise need help

Hey all, I have a 2000 Ford Mustang 3.8 V6 that has alot of people scratching their head.
If anybody can help me out with this id really appreciate it.
When my car is in drive or reverse only, i have a loud tapping noise that only occurs when the car is at operating temperature and when given a little throttle. Rev it in park or neutral....no noise. Let it idle in drive or reverse...no noise. If i drive over 30mph, it cannot be heard. If i add tension or remove tension on the belt, it goes away. If i remove the belt completely it goes away. People were telling me to check all the accessories and pulleys and my girlfriend has an 03 so i swapped our idler pulley and tensioner & tensioner pulley and that wasnt it. I swapped our belts as well. My water pump is brand new so i know thats not it.

anyways, today i decided to do the pull one plug at a time trick and when i pulled plug #2 the noise went away...im really confused and stumped here guys, any input would be appreciated.
thanks.
 
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