Rattles at low reves only

You have low oil pressure. Remove the oil pan and replace all the internal and external orings

What kind of vehicle ? Which engine? Need more info.

This type of problem is hard to diagnose without hearing the actual sound in person, but it's possible you're hearing the oil pump chain.

Does it sound like the rattling is coming from the front, bottom of the engine?

There is a chain under the clutch basket that drives the oil pump. Over time, the chain stretches and gets loose, requiring replacement. It goes without saying that failure of this chain will cause catastrophic damage to your engine, so it should be checked if it sounds like it's on its way out.

Here is a link to info on accessing the oil pump chain on an ST1100:
http://www.st-riders.net/index.php?topic=7695.0

balance shafts are in the block ,not the top end
could be cam shaft bearings or a failing cam lobe
could be a cam advance servo unit that is faulty at rpm and knocking from low oil pressure supply

could be the tappets/risers that noise is comeing from

ck balancer on front of crankshaft

temp sending unit or low on water

sounds like a auto box fault to me,,if its doing nothing and wont rev,past,,2000rmp this is about all it should rev at when doing a "stall test" the gear box may be locked up if it is and you keep reving the guts out of you will burn out the gear box up,,,have you tryed jacking up the rear wheel of the car to see if its not the diff that locked up

from what you describe it sounds like cam chain,use a long screwdriver ( or something like ) as a stethoscope and touch the engine with metal end, put the screwdriver handle to your ear and locate where the noise is coming from ( mind you dont get burned or anthing caught in moving parts )

Rattle generally means detonation, which will destroy your engine. How quickly that will happen depends on the severity of the detonation. You can cure detonation by retarding the timing, cooling the intake charge, using higher octane gas, or lowering your compression.