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I've had 3 different major issues with mine over the years - a spun, knocking rod bearing, a loose, broken cam gear, and several instances of flat lobes/trashed lifters.
Generally, the flat lobes didn't make any unusual noises, the engine just started to develop a somewhat subtle and hard to track down miss. Sublte and hard to track down because it would still idle perfectly evenly, just one of the cylinders would be weak under power, when the flow required to run at that power level exceeded that amount of air that could flow through a partly opened valve. So it would feel like it was missing when you drove it, more ragged the farther the pedal went down, but be perfectly fine when you stopped the car and tried to pin it down. The one time it was an wexhaust lobe, that cylinder began developing a nasty backfire too, as burnt fuel wasn't getting sent out the backside, but was backfilling into the intake manifold. In all cases, once the lobe was flat enough to make a noticeable difference in how the motor ran, it was glaringly obvious with the valve cover off, cranking the motor around made it stand out like a sore thumb.
The spun rod bearing/rod knock made a deep clunking noise, very serious sounding heavy metal lunking. And it came and went, depending on what the motor was doing (differing loads on the piston rods). In general, it was pretty quiet at idle, fairly quiet when speeding up. And noisy on the overrun - when the engine was revved up and then the throttle was suddenly shut. Really loud knocking as the motor slowed down, regardless of RPM. And I found that I could get it to knock very audibly at idle by pulling the sparkplug on the affected cylinder. No noise, clunk clunk clunk. After that, I drained the oil and swa lats of sparkly stuff. *wallet let out a little scream*
The separated cam gear really matched your description of the motor soundling like a diesel. Just like a semi diesel idling. Not quite as deep and serious sounding as the rod knock, but certainly more persistent. Clanking constantly at lower rpms. Clanking when speeding up, clanking when slowing down. What I did notice, however, is that it would actually quiet down and go silet over a certain RPM. Which makes sense, at low rpms there's not much load on the cam shaft, the torque loads on it go back and forth as each cam lobe first strains to push a lifter up, and then is pushed forward by the lifter on the back side of the lobe. This back and forth torque load yanks back and forth on the separated gear, and makes it rattle and clatter back and forth because of the angled teeth. When the motor is revved up, the drag from the oil pump increases, and the lifters start t0 push the cam forward a little less (momentum, inertia), and at some point the cam goes into a constant turque and this quiets the broken gear down because it's in a constant pull.
Disagnostic steps I'd do if I were you:
1) Easiest, pull the valve cover, pull the low tension wire off the coil, and crank it around and watch each valve go up and down in turn. See if there's a noticably short one. You'll need new lifters and a new cam if so, and the head will need to come off.
2) Pull the timing cover off the front and check the timing gear and thrust plate (the brass or steel bearing that bolts to the block in between the cam and gear). Taking the radiator out helps a lot. Unless you drive like a fiend (heh) a replacement fiber gear is probably all you need to go another 100K miles of driving.
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'63 PV544 rat rod, '93 Classic #1141 245 (now w/16V turbo)
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