Catalina 350 International Association

We escaped our self quarantine at home by doing an overnight sail.
Fun but too short.
We were again reminded of a strange noise heard only while at a dock;
Sound can be best described as a beep. Seems to be coming from under the forward bunk area. It's consistent in time so not likely a rigging or mechanical issue
I checked the bilge, nada
Turned off all electronics and the whole 12 volt system-nada
Turned off the water and purged the system, nada.
Put her in gear, thinking spinning shaft, again nada.
Does the depth sounder emit a noise? After overnight with sound continuing, I swear i heard an echo!
I don't know what forum this goes to but any ideas would be appreciated.
Stay safe! Thanks in advance.

With the instruments (depth meter) off, can't see a depth meter making any sound without power to it?

Any high water alarm in the bulge? Electronic float for bilge pump?

Did you turn off the battery switches? Any change? That would rule in/out12 VDC devices.

Yup
Turned off main battery switch. Let pressure out of 12V water system. Still hear it.
Sometimes we hear tap...tap.. tap sound but that's normal in Florida. Usually fish just "cleaning" the bottom.
This is a muffled beeping sound.
More ideas?

This may seem way off the wall. At the dock only? Not when anchored? Is there a lighted flashing beacon nearby? We had a mysterious sound that seemed to be coming through the hull. Pulsing every 2 seconds. Turns out the pulsing frequency matched the rate of the flashing light on the beacon which was about a mile away. Never heard it again after leaving that anchorage. If not that, check with your dock neighbors. The sound may be coming from another boat.
Rick
Sea Breeze
Tampa Bay

Thanks Rick
We will lookout for nearby beacons. The noise does seem to be in the boat, but maybe not.
Also, checked again yesterday, no noise at our dock.
Appreciate any other ideas.
Stay safe!