Ghost in the machine?

Have something curious going on with my 69 Superbee.

In the quiet of the garage, car just sitting with no key in the ignition, about every 15 minutes or so you can hear one of the horns buzz for a fraction of a second (the one closest on the circuit to the harness).

I tightened all the grounds on the horns, firewall, engine. I changed the horn relay. No difference.

Thoughts as to what is going on? The horns work great as normal, nice clear sound.

Maybe a short in the horn button under the steering wheel.

It'd have to be in the horn relay or in the ground circuit back to the col as Stump said. New relay so it'd have to be in the col or from the relay to the col. I would seperate the bulkhead that houses that circuit for a quick visual then plug it back together then seperate the dangling column harness & if it stops then the prob is in that harness from there (the pull apart connector) up to the horn button

Does your car have a clock in it?

it does have a clock in it, one that works sometimes. ?

Well, that noise isn't the clock re-winding instead of the horns making noise, is it? If it happens with regularity, I bet that is what it is.

Interesting idea. The noise is very, very, regular. Something that I thought was quite weird.

Good ideas all, I will try and inspect everything this week.

I agree with the clock thought. Unplug the horns and see if it still occurs.