Loud noise in the cockpit

This case was during a real approximation Tacan-GCA and flying within a cumulonimbus, the pilot was being whipped around as well as his airplane in the center of a storm with hail and lightening, when suddenly a warning siren sounded.

Immediately he declared an emergency when he assumed the loss of engine oil with the  known loss of thrust because of the malfunctioning of the Nozzles. Besides the warning signal, there was a very loud noise in the cockpit and he did not where it coming from but it was going away as he was landing.

Once on the ground they found out that the reason for the noise was the broken glass from the infrared visor and that the siren was not from the engine oil, even though they were very similar.