this does sound like pre-detonation though westy, which could be caused by a whole ton of things such as a done in piston ring to a done in spark plug which may or may not be firing correctly, take them out and have a look at them and let us know the condition of them will you? if they are ok, check your oil dipstick. if you notice excessive amounts of petrol smells in your oil that would mean that the piston ring is gone, which could also be a symptom of far more serious issues such as the ones you are worried about. if you have a stethoscope which you can get from any motor factors, you can listen in on the engine and trace the source in terms of a general location before you spend 500 on a strip down to find out that it was a dodgy spark!