Hollow knock coming from Under valve cover driver side

Hi guys, first time posting.. I have a 2001 double cab 3.4 automatic 4wd with 178k. Oil change every 5K on syntetic. Timing belt ,water pump ,drive belts replaced at 90k. Idle air controler replaced at 150k. Spectre cold air intake . Flow masters cat back exhaust. Original owner. I recently heard a hollow knock coming from inside the valve cover on just the driver side. It seems to get louder when RPM are increased but still remains to be hollow. NO check engine light on or engine malfunction light on. And yes all bulbs on the dash lights up at start and turns off. My truck drives fine without any problems. And idles fine But, the hollow knock seems to be there all the time. I do not think it is an exhaust leak because, it does not sound like the same spitting sound I normally hear. Any clues or speculations it might be before I take it to the shop so I know what the mechanic might say to me while he listens to the sound and charges me $120 /hr.