noise

Lifters will be loud either idle or above idle, load won't matter much - but "generally" a lifter will be more noisy at idle.
Main bearings, rod bearings, that's a different matter.
Rods can tend to quiet down when put under very heavy load because the pressures of combustion, and compression keep the rod pushed against the crankshaft.
Main bearings will be more noisy under load due to crankshaft flexing.
There are exceptions of course, but those are generalities.

Other possible sounds - fuel pump, but typically makes noise regardless of load, water pump, again, not really dependent on load, but may be on speed due to accessories, belts, etc.
Broken valve spring, something in combustion chamber which hits head as engine speed rises and rod stretches a bit, flexplate, flywheel bolts, cracked flexplate, there are other things.....
Lifters are higher pitched and sound like they come from the top, as the sound goes up the pushrod to the rocker and echoes under the valve covers, rods and mains sound deeper.

If it's an automatic, see how it changes when put in gear, or how it changes when put in gear holding the brake hard and pushing the gas. If a stick - does it matter if in neutral with clutch in or out, etc.

Sometimes a very long screwdriver with the handle held next to the ear and the blade put on the engine close to various sound sources can help pinpoint it, or a mechanics stethoscope can help.

Knocking? Could it be preignition knock which will occur under load if the timimg is not right. Just another possibility.