My 5.2 has roughly 90,000 miles on it. i notice that when it warms up there is a weird knocking noise, it almost sounds like a flapping noise that comes from more on passenger side of the motor. I notice it more inside the cab than outside. When its cold it doesn't do it, only when it warms up. Once I start to hear it I can turn the truck off and turn it right back on and it takes about 10 seconds to hear the noise again...any ideas?

 

It may be a few things but you may want to try1qt ofLucas oil stabilizer at every oil change it will help with the knocking. I would definitely locate the exact location of the knock. I have heard on here that an old garden hose makes a great stethescope to pinpoint engine noises just trim it down to size.