Salam to All
Agreed ,,,,,,,,,,,..........
check all the engine foundation's specially near the gear box area for 2nd problem its common in plastic/fiber made intakes manifolds,
(because of CNG back firing) inside of intake manifolds some plastic plates are designed for control & divert the air flow for all four pistons/cylinder's.......
if your CNG system is not properly tuned or running with old (dirty) Air filter or week spark plugs.......
result back fire inside the manifold,,,,,,,
try to replace Intake manifold with new part sound will gone,,,,,