To mount a float switch in the bilge, I drilled a small 2.5mm pilot hole (well several small pilot holes) in the platform immediately aft of the deep well in the bilge - underneath the engine. After drilling one hole there was a discernible whistling noise - like a little release of air for maybe 3 or 4 seconds. No visible issue came from the hole (phew) and, as I say, the whistling soon stopped but it has left me wondering what I drilled into. Is there perhaps a cavity in the encapsulated ballast? Was this just ambient air pressure - hot day yesterday? Some byproduct of hidden corrosion? I observed said hole for several minutes to ensure there was no seepage then inserted the screw (with a small blob of sealant) and completed mounting the automated pump equipment. The boat still seems to be afloat 
.
Anybody any similar experience? Anybody have a reasonable explanation?
Many thanks.
Hi Simon
I think you hit it head on. You drilled into a small void. Under the engine is far aft of the ballast though.
One day you may want to open up the hole and add a small dose of 2 part closed cell foam...but I'd not worry about it.
My electric bilge pump is mounted on an L shaped plastic [HDPE - High density polythene] fabrication secured, at the top of the 'L' to the side of one of the engine bearers. It's easy [ish] to get at and remove the pump which sits on the bottom of the 'L' It's so set up that the pump draws from the deepest place.
Peter
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