I have a 99 XLT, 5.4 and it has a knocking sound from under the cab. It sounds similar to a rod knocking but not as loud. I do have an exhaust leak but I know what that sounds like and this sounds deeper. This is worse when cold but seems to be there most of the time when idling. It sounds like it is coming from the cat. Does that seems reasonable? I attached a sound file.

 

added a sound file

 

Sure sounds like a knock to me. I had a knock in my 4.6L (or so I thought). I also had an exhaust manifold leak. Once I fixed the manifold leak, the knock magically diappeared. Maybe you'll be as lucky. Is your exhaust leak from a broken stud or studs???

 

I can't open the file, but usually the cat rattles when a heat shield is lose. A large radiator clamp can be used to tighten it up. Just wrap it around the whole thing. An exhaust leak usually sounds like a tick on every car I've heard it on.

 

The knocking doesn't really sound like an exhaust leak or a rattle. I have heard both on my sons truck and this is different. I was recording near the cat on the left side. I will try to use a stethascope to see if I can narrow it down.

 

Any luck? i have been trying to fix it on my truck for ages!!!

 

I still can't tell where is it coming from. I used the stethoscope and I can hear some faint knocking from the engine block but it is much louder outside the block. The sound comes from near the block and the exhaust manifold but doesn't sound like an exhaust leak since it is more a knocking sound and not a ticking sound.