EMI - Electromagnetic Interference / Noise
Updated April 18, 2009
I was experiencing noise in the radio whenever I turned on the blower fan with the heat or the A/C. The frequency of the noise would change with the speed of the fan. I traced the problem to a poor connection where the power and ground hook to the blower. It probably was the ground.
Crutchfield carries many noise filters.
Other Common Noise Filters
The top filter is for alternator noise. It's #14 wire wound around a toroid coil wired in series with a 2200uf capacitor to ground.
The second filter is just a coil and the third is a .01uf capacitor which would be wired across the power to kill higher frequencies.