is it a sudden thing or has it gradually got worse? if its been gradual you may need to re-do your valve clearances.
it may also be a more major problem. take the cam cover off and have a look. if you have a decent camera take a pic with the cover off and post on here.
could be a ratty in the cams so chek this out http://www.toyotagtturbo.com/forums/showthread.php?t=41558 thats if you got a new headgasket done
could be your oil pump on its last legs if it used to do it on start up and then was ok. does your oil pressure switch work? does the light come on then go off when you start the car?
could be many things
if it is that rattle cams it wount harm the engine just have a look when you take the cover off
if that light isnt working the previous owner may have removed it to hide a problem.
turn the ignition on, if the light isnt on then remove the connector off it (its a little bit fiddly) and touch it on the engine somewhere, that should turn the light on, if it does the oil switch is knackered. if it doesnt then the bulb may have been removed, the wire may be damaged somewhere or the sensor could be knackered.
sort that first as that could well be the cause of your problems
basically. yes. you just need to touch the connector on something metal that is grounded to the battery - terminal, anywhere on the engine usually does fine.
checking the oil light may point you in the direction of what the problem could be. it may just need a new bulb. replace that and start the car and if the light goes out after a second or so afeter starting then all should be fine with the oil pump.
its a problem so may as well sort it, then move on