I've listened to your you tube recording , and I am sure that there is the sound of a loose tappet there , it may be caused by worn bushing, worn shaft or bent pushrod , either way , the engine IS capable of running quiet when its all adjusted correctly.
My 216 cannot be heard at 40mph (over the wind noise and engine fan noise :)) so I think your truck can be made much better and I think you should get right in there and fix it .
Also as enginenut says above .
The shop manual has a good write up on correct adjustment process.
