As ScrawnC says, if it is more a rattle that comes in under load you could be luck as it could be engine 'pinking'.
Cause for this is typically ignition to far advanced coupled with poor/low octane rating fuel.
Has anyone messed with the timing recently?
Check to see your distributor hasnt been left loose.
IF I thought it could be pinking, before I investigated further (and went to any expense) I would add some of the high octane fuel additive to the tank (or fill with premium).
Then if that didnt fix it I would mark the distributor position (scrape the metal or mark with a marker pen or something to mark current timing position) then loosen and rotate the distributor a little, tighten and try start the engine again. (move only a mm at at a time, first in one direction for a while and then in the other (I guess if it is pre-ignition (pinking) you will get better result by rotating clockwise).
(I tend to roatate the distributor whilst the engine is running, but as there is risk of shock from HT leads or even a cracked dist cap, I reckon if you are new to this proceedure that you do it the safe but slower way. Stop the car make your change then start the car.
Good luck mate
G.