With you going the stated speed, and having changed out what has been done, my guess is that you have a short"ing" wire somewhere in your alarm low side to ground (battery -). If you have a control box, like the Quicksilver/Mercury 2000 or 3000 series and your horn is inside the control box, the position of the throttle/shifter lever could be rubbing on an exposed (insulation chafed off) ground (battery -) side of the horn and causing your problem.
If the horn is wired under the dash, then the problem could be in your wiring, either between the horn and the engine or in the engine proper. The 8 pin connector pin 3, tan/light blue stripe) wire is your ground wire for the horn in the wiring harness. Inside the engine the color codes would be tan for OT sensor, tan/lt blue stripe in a Y connector, or light blue for oil level sensor if the wiring is still in the engine....since you said you no longer have oil injection...assuming the tank has been removed. Any of these wires vibrating at your mentioned speed and touching the exposed metal case/part of the engine could sound the alarm.
If your engine has the oil warning module, some earlier engines had such...follow the light blue wires that were connected to the oil tank to find it, (to the rear of the oil tank and down low just above the lower cowling) it could have an internal problem...just disconnect it.
