Firstly, anything is fixable if you have enough time and money. As to what is causing your rattle in particular...it could be any one of a number of things (in no particular order).
a) Bearings b) Flywheel c) clutch (if it's a manual) d) timing chain tensioner (fairly common, and not too tough to fix) e) timing chain f) First signs of the harmonic balancer letting go g) Bearings in one of the pulleys on the front of the engine.
And I'm sure I've missed a few things.
Mine's got a rattle and I can't work out what it is, but then again, I haven't had the time to open the engine yet. Does your car sound like marbles too?
Bite off more than you can chew, and chew like hell - Peter Brock (1945-2006)