Like Tony said, a stethascope is a great buy to pinpoint noises. Sometimes when removing the sparkplug wire from a cylinder and the noise goes away or lessens by a bit is the wrist pin on the upper connecting rod going bad too. A rod knocking noise increases with engine rpm and is a deep noise and not fixable without pulling the engine.
Any car can run low on oil and still run fine, in fact most cars do use oil and it is not uncommon to have to add a quart or two on older cars, but it is an indication of a leak or oil burning. Any car that lets the oil level get too low and lets air get into the oil pump is going to ruin the engine. The Toyota actually has a very good oiling system and a very good oil pan setup. I know that when I rebuilt my Supra engine and replaced the oil sending unit, I would have around 60 lbs pressure at startup and after warming up in would drop some. But my oil pressure would always follow the 10lbs per 1000 rpms. Although it does drop down to below the 10 lbs at idle when warm, the red low oil pressure light never came on.
You might look at the crank pulley/ vibration dampener as a source of the noise or even an engine or transmission mount the might be hitting the frame while under load, but any mechanic can easily tell you if the noise is a rod bearing. It is a distinctive sound heard very well under acceleration. It is harder to hear at idle unless it is really gone bad.
Try to get someone you can trust to listen to the engine. Don't let just anyone try to make the diagnosis, because an engine rebuild makes the mechanics eyes gleam when thinking about the money that can be made.
Russ
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