Getting tapping noise when letting off the throttle

My ford e350 (exambulance) odometer 83900. It just started making a tapping/knocking type of noise in valve train area. Starts fine, no sound at idle when driving/throttling from a stop or on the highway an obvious loud tapping ( as if a ballpein hammer tapping on valve cover area). The other thing to note is letting off the pedal noise stops and or stomping on pedal noise also stops? Thanks. Tyler