OCC hatches are double layered.
The outside is one surface and there is air between it and the inside surface. You may have been hearing air leak out from between the surfaces of the hatch cover.
The bulkheads are tight enough to make the bow pucker when the temperature goes from really hot during the day to cold at night. Worst thing is to put the boat away for the winter on a warm sunny day with the hatches tightly sealed, and leave it that way over the winter outside. Another is to leave the boat on top of the car in the sub with the hatches battened down tightly.
There may be some merit to the idea of drilling a small breather hole in the middle of the bulkhead. If the air pressure does become critical inside a compartment that is tightly sealed, it could find its way out through a bulkhead.
I never put my important stuff in the
day hatch. I put it in the rear hatch which I never open unless something really unusual is happening.
I have never kayak camped because I got into the sport when my wife was seven months pregnant and I have never left my family for more than a day. Maybe we will coast hug lake champlaign this year and do some camping together, In which case my foster shadow will be of some real use.
Equalizing Pressure
Nigel himself recommends drilling a tiny 1/8" hole in the center of each bulkhead to ‘equalise’ pressure. Before I did this, my front, rear, and day hatches on the Explorer would alternately ‘Tupperware’ out or in with expansion or contraction, depending on the temps. Now, it’s unaffected by temp extremes, and I no longer have to ‘burp’ the hatches. I’ll check the seam between the rear hatch sections on the QCC-thanks; the first one not only leaked into the compartment, but the hatch itself filled with water.