After you've checked the key steptoe, if it turns out to be intact, it would help both your question, and our archive, if you take a look in the crankcase. That way everyone will know what it was, even if the engine is beyond cost-effective repair.
To help clarify the issues, severe detonation usually develops suddenly after an adjustment or mishap and makes a sharp clacking sound that is rather distinctive when you've heard it, but hearing it is usualy followed immediately by a broken piston and no further sounds at all. Loose big-end bearings normally start off with a slight rattle which is less noisy than normal Briggs tappet noise, but it gradually gets worse if you don't attend to the cause, which if it is an older engine, is nearly always lack of oil in the sump. If an irresponsible owner just ignores the noise and keeps running the engine at low speed and low load, it may advance to the stage of a loud rattle that dominates other engine sounds.
An anecdote that probably won't help but will make me feel as if I've explained. Long ago my grandfather had a little OHV Morris Minor. One day my father happened to drive the car, after no one but grandfather had done so for months. There was an appalling clatter of big end bearings - really loud - and no oil pressure. My father remonstrated with his own father, saying "Surely you must have noticed it developing that noise?" Grandfather replied, "It's all right if you're careful not to open the throttle much". The previous owner of your Quantum may have been a person like my grandfather.
A supplementary anecdote, about high speed detonation and how fast it kills the engine. I was driving on a Melbourne urban freeway some years ago when a Brock or HSV Commodore went past, flat out in third I think. Just as he passed me the car disappeared in a cloud of exhaust smoke - he instantly shut it down and coasted into the emergency lane. Looked like he had just been working on the distributor - that's how it usually happened, some genius decided he needed more centrifugal advance, and made his first test of the concept at full throttle, maximum engine speed. As I explained to my wife at the time though, in those days of fairly cheap, carburetored homologation-model racing Commodores, the young guys who bought them got most of their jollies from bragging to each other about their blow-ups. (High speed detonation normally breaks at least one piston within one to two seconds, before most people can identify and react to the sound.)
