If the flasher is flashing quickly, that generlly indicates too much load on the flasher. Most standard flashers operate as a thermal breaker. Current causes a metal part to heat up, it bends, breaks the circuit (blinks off), cools off, flexes back to normal shape, reconnects the circuit, light goes on, repeat. The more current, the faster it blinks. the less, slower or not at all.
So flashing quickly could be a bulb too big, too many bulbs, or most likely, a wire touching ground somewhere (shorting out). You see this happen frequently when a trailer with lights is hooked up to a standard flasher and they blink fast. i.e. too much load, too many bulbs.
LEDs have almost no load, therefore draw very little current, and therefore generally the flasher doesn't know it is there, and does not blink, or very slowly.
To fool the flasher and make it think there is an incandescent bulb in there, a resistor is placed in parallel with the LED, generating more load. Now the flasher is happy and flashes. It thinks a regular incd bulb is in there. By my caluclations, for a 6 v system, that resistor should be about 18 ohms, 3 watt. That would generate about the same current as an incd bulb.
Putting a low value resistor in series with the LED would have almost no effect at all since there is little current and therefore little voltage drop across the resistor.
BUT your flasher blinks fast with an LED. Something is off. That is the opposite of what should happen. I would suggest getting it to work correctly with a standard incd bulb, then substitute the LED. Or try LED on one side and incd on the other and see what happens.
An electronic flasher does not use a thermal strip. It has a counter that breaks the circuit at a number of milliseconds (usually you can set this time with a little screw). And no, it generally does not make a sound, but there may be some out there that do.
