Some semi-random ideas:

Timing chain jumped a tooth?

Fuel system issue?

Blown head gasket? What does the smoke smell like, and how are your coolant and oil levels and conditions?

Other internal engine problem? Have you had any more unusual noises after that initial bang or pop?

Also, what's the patient history? Miles? Any maintenance done recently or know to have been deferred by more than a little? Incidents/mishaps? Ever vegged?

__________________
Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.