No it's Bernard hitting his head against the bulkhead again, which makes a nice hollow BOIIINNGG sound
My best advice early war would be:
In daylight, stay submerged and be very careful with the scope, only attack if you're going to lose the target otherwise
At night you have a lot more leeway and the advantage over the escorts if you stay surfaced, at night, only dive when they've seen you and have opened fire,
as to torpedo attacks, don't be afraid to use your TIs at medium speed from 4km or 5km out
they will still usually spot them too late to do anything about it.
for big (unrealistic) tonnage do the one eel one ship thing... at least in GWX. in stock SH3 your best bet for this would be C3 cargos, fire just when the bridge hits the vertical crosshair, this splits them in half 9 out of 10 times
oh and sink every single merchant you find... no exceptions
