I've had this diesel like sound since I've owned my Disco. Let me preface by saying that I've put 11,000 miles on it and the sound hasn't gotten worse.

I should also add that the previous owner had the oil pump fail. I spoke with the mechanic who replaced it and he said that the pan was pulled to check for bearing damage and none was found.

I also changed the head gaskets and didn't notice anything out of the ordinary. As a matter of fact, the top end is now so quite I can hear the fuel injectors. Something that I couldn't hear before...

Now that I've an oil pressure gauge in the cab, I'm keeping an eye on it. Cold pressure is 30/60 while hot is 20ish/50. The gauge is an off the shelf Autozone model. So while it might not be that accurate, it at least lets me know that my pump is working.

With the background out of the background out of the way, my initial thoughts (or at least optimistic tendancies) makes me want to check outside the engine first. If I had some serious bearing issues, I'd certainly have known it by now.

More about the noise: can't really here it while in the cab unless I'm starting up a hill or the tire's climbing over a bump/obstacle. At speed I can't hear it at all because of the road noise. At idle (in park) you need to be under the engine to really hear it. The diesel like knock is about 1/6 of engine RPM, so a 100 or so taps per minute at idle.

What should I be looking for?

 

That's pretty good oil pressure, however, I had a an '04 that made an awful noise at start-up but generated fantastic oil pressure almost immediately. When torn down, the oil pump was in six or seven pieces. The crankshaft was toast.

The diesel like knock is about 1/6 of engine RPM, so a 100 or so taps per minute at idle.

Valve train, perhaps one lobe.

 

Wouldn't valve train noise be more evident from the top?

The only thing I hear there are fuel injectors.

I'm hoping for something along the lines of bad cat or perhaps an exhaust leak. Anything but internal engine problems.....

 

get under the truck and bang the cats with a rubber mallet.