So I drained my air tanks free running my 1970 m35a2. After draining the tanks, I also replaced the light switch and discovered that the wires were broke right before the flasher Relay. After putting it back together, I started up the truck and after 20 seconds or so, it started howling super loud! I could only get it to stop by turning the truck off and bleeding the air tanks. I have never heard the horn on this truck but I was wondering if it may be it? Any suggestions?