sounds like marbles in the motor
that noise after changing the rear main seal and oil pump could be the crank bearing caps weren't tightened down correctly, or a bad oil pump that isn't sending oil to the rocker arms. i've heard of people putting roller rockers in and getting an alarming noise on the first start up as they were getting their first oil bath, if the stock rockers were dry then you would get a noise similar to that.
and the valve covers are easy to take off, just tie the wires/hoses out of the way and take all the bolts out and they pop right off. if you take the valve covers off, don't forget about the ground wires on the passenger cover when you put them back on.
did you notice what the oil pressure gauge was reading when you cranked it up?
wait a minute, isn't there a keyway (i think that's what they're called) on the front of the cam and crank that holds the timing gears in place? if it started then the timing ought to at least be close, but one of the gears could be loose couldn't it?
and the valve covers are easy to take off, just tie the wires/hoses out of the way and take all the bolts out and they pop right off. if you take the valve covers off, don't forget about the ground wires on the passenger cover when you put them back on.
did you notice what the oil pressure gauge was reading when you cranked it up?
wait a minute, isn't there a keyway (i think that's what they're called) on the front of the cam and crank that holds the timing gears in place? if it started then the timing ought to at least be close, but one of the gears could be loose couldn't it?
Please tell me it didnt sound like this 
http://www.youtube.com/watch?v=IFZYAbcBr80
I'm still hoping this is normal and will be fixed with a new plenum plate...

http://www.youtube.com/watch?v=IFZYAbcBr80
I'm still hoping this is normal and will be fixed with a new plenum plate...



