got an 82 280zx, when i bought it, it had no stereo and the original stereo was no where in site. After much trouble shooting and staring at a wiring diagram for an hour, I installed a new alpine stereo in the car. I also installed new infinity speakers all the way around. If I turn on radio with car off, it works perfectly, sounds great. If i turn on the radio with the car running, it makes an odd hissing type noise, but once the car gets to full operating temp, the hissing slowly fades out and the stereo sounds good again. I was thinking it might be something to do with the alternator, the hissing noise gets louder with rpm increase. I just installed a brand new alternator though. Anybody have any thoughts or experience with this problem?

You may need to put a resistor in line somewhere to supress the electrical noise.

Resistor won't do it, will need a capacitor in the power lead. Not sure this will help since you mention the noise going away once the car is running at operating temp. Give the capacitor a try, I don't know the specific type but the sales guys at your local radio shack or stereo shop should be able to recommend the proper capacitance rating. Hope this helps..

does the capacitor need to be on the constant 12v or the switched 12v?

Radio power filter, or noise suppressor is what you need (capacator + coil + resistor) it's not that the car is warm, it's the battery gets a full charge and the alternator isn't putting out as much current.

oh, i see. well that definitely makes sense

I never heard of the idea being used on a car but with two way radio equipment sometimes they use a small ferrite doughnut looking thing that the power wire is routed through a few times.

"Radio makes funny noise"

I think it is called "RAP".

The noise suppressor is (or should be) mounted with the ignition coil, on the mounting bracket itself. It may well be disconnected, missing, or simply worn out...