I think it is most likely the rigging vibrating due to resonance--caused by the turbulence of the wind.
However--there is another possibility, one that is more concerning--and that is a dragging anchor
on a gravel sea bed
. If you are using an all-chain rode
, add a kellet, also known as an angel--which alters the angle at which the rode pulls on the anchor and can, in good ground, effectively double the holding power
of an anchor. It needs to be shackled on to the chain at about one third of the total scope
, taken as measured from the anchor. I used a kettle weight which I bought at a sale
for ten dollars. Previous to that, a length of railway iron track with a loop welded to one end. It needs to be about half of the weight of the anchor to be super-effective, but any weight at all will be helpful. Not only does it make for better holding--it makes for a soft and gentle anchoring
I have on two occasions drifted during a blow. Both times it was a dragging anchor. Now I take anchoring
extremely seriously. A kellet will give you the most holding for the total mass of all anchor tackle carried aboard.
Still--I think it sounds more like the rigging to me. It depends on the violence and period of the vibration. Resonant vibration can be quite severe, but it is more or less regular in its pulsation.