low knock noise in eng not fast as eng rpms

try putting a higher octane fuel in it to see if it still knocks if so your timing belt/chain (I can't remember which is in that truck) may be getting worn out or stretched just a little. Another option it could be that your cylinders are not getting lubricated enough, which could be from low oil or just lack of pressure since it doesn't happen when your driving. Try adding Lucas oil stabilizer it may help thicken it up or some product that is similar.

thanks cory