help on lightweight

Ive suddenly developed what sounded like an exhaust blowing, and a lack of power, but on inspection its noise coming from the air inlet. The engine ticks over OK but 'growls' when picking up. All plugs are firing but if I remove No2 or 3 HT lead it doesn't seem to make any difference to the tick over. Ive swapped plugs to no avail. This applies to running on LPG or petrol.
Ive got a feeling that I might not want to hear the answer to this!!!!! or am I just missing something simple

Andy

It sounds like the head gasket might need replacing. It's not too difficult to do.