1989 Chevy Caprice Knocking sound

My 1989 caprice classic V8 305 TBI is making a knocking sound like something is loose mostly hear the sound when in Park or when idleing holding the brake in drive but while accelerating stepping on the gas the sound seems to be gone or very quiet. Car was recently tuned up passed smog and everything. Ive checked the the vavles and adjustments seem fine. Car still drives with power and without a problem very smooth. What could be that knocking sound?