Radio noise

my 1991 nissan maxima is a perfectly preserved gem (low mileage, mint condition), however when the engine is turned on a terrible noise comes out of the radio speakers. It is a high pitched squeel accompanied by static. Any ideas what might be causing this, and how do I fix it (is it the radio, or the speakers, or something else??) Thanks

Since the “interference” occurs when engine is running I think the most likely culprit is the alternator. Now did the alternator start making more electrical noise or has the radio lost it’s ability to “ignore” this noise?

As a test you can try shielding the alternator with aluminum foil,this may give you a clue, you could try a replacement “test” radio (part of my tools when did this kind of work for GM was a known good test radio,many people don’t have this option,its use helped me diagnosis many problems).

I avoid saying to concentrate on the antenna as all is well with engine off.

My feeling is with the alternator.

If it sounds ok with the engine off, but noisy with the engine running, that points to shielding or the alternator.
Check the alternator for any a/c output. There’s a filter capacitor in some circuits from the alternator. Look for the capacitor on the wiring diagram. It may not be working (filtering).

Another way to see if the trouble is coming from the alternator is to disable it briefly for a test. By pulling the fuse for the alternator or disconnecting the small wires running to it will cause the alternator to stop working for the test. Don’t mess with the large output wire of the alternator since that wire is hot to the battery and you don’t want to short that wire to ground.

What you are describing sounds pretty much like a classic case of a bad diode or two in the alternator. This will also cook your battery if it goes on long enough.

Find a good shop to test your alternator for AC output, which it should not have. The quick test machines they use at auto-parts stores won’t always catch this problem. I’ve seen it.

By the way, does your Maxima have the Bose system? I used to have an '89 Maxima with that setup, and there are amplifiers built into the speakers. Those can be bad and there is a place that will rebuild them.

I don’t know if there are condensers in the distributor of your car, if there are that is a good place to look also. Does it increase with accelleration? if so spark plug and coil cables.