Land Rover Repair: knocking at idle, piston slap, crank shaft

Question
i have 1995 LWB classic,I am going through this truck which has 128 k on it, it was sitting for a few years according to the guy i bought it from. Anyways, I was going through this rig prepping it for a trip in a week and i noticed a low pitched knocking noise at idle. I got under it but i could not pin it down, it sounds like a bad crank bearing sound but when i go to the passengers side i dont hear it. I go back to the driver side and it hear it. it also sounds like its coming from the front of the motor. I could not hear anything while under the oil pan........so what is this low sounding knock? I had an 81 chevy PU with a 350 that had bad bearings in the crank shaft, this kinda sounds the same but its not directly under the motor. I can rev it up and it disappears,but when it goes back to idle the random, in and out knock is there. Its my first Rover and I have no idea what its suppose to sound like.....is this a fuel issue? octane? i have 90 in it but does it need better than that?

Answer
It's possible you head a knock from inside the motor but it's more likely the knock is the elements moving inside one of the catalytic converters.  Look again and see if that's not the case.

We do sometimes see old Rovers with piston slap, and you get the occasional bearing noise, but cats are the most likely bet.