Loud Viberating Rattling on startup -DIESEL 1.9DL 1999-

[CheekMaster]

Have a 1.9dl polo 1999

When i attempt to start the car, sometimes a loud viberating rattle comes up, may be from the gearbox, however, if i leave the key in ignition for some 30secs (maybe after a few times) or so, it starts up and runs fine.

this sound is way above the normal. imagine smashing a hammer against the engine, thats how loud it is, it only happens for a second or so, then the car shuts off

Any ideas on how i can remedy this ever so frustration problem???