AM radio picks up "radio frequency interference", (RFI) which means anything that causes voltage spikes, like lightning. In this case you've already identified how you can make the noise occur, but we need additional observations to narrow down the cause. Does the noise occur only when the engine is running or any time the ignition switch is on?
If the engine must be running, the generator is a common source of harmful voltage spikes on GM cars beginning with 1987 models. To verify that, unplug its small connector at the rear / side, then run the engine. If the noise is gone, suspect the generator has a failed diode. That is caused by those voltage spikes it develops. To reduce the high number of repeat generator failures, always replace the battery at the same time, unless it is less than about two years old. The battery is the key component in absorbing and damping those voltage spikes. They lose their ability to do that as they age and the lead flakes off the plates.
Also consider the anti-lock brake hydraulic controller. You may want to temporarily remove the two fuses for the ABS system. If the noise stops, look for a rusted / cut / forgotten ground strap between the body and engine, body to hood, etc.
Observe what the noise sounds like. A generator produces a high-pitched whine that increases in pitch as engine speed increases. Ignition system noise is lower-pitched, and tends to get louder when engine speed goes up. Static that sounds like a weak station, but it changes when the car isn't moving can be caused by antenna problems. GM refused for many years to give up on the disaster that put the antenna in the windshield or rear window. Those tiny wires break from vibration and extreme temperature changes. If you have the more reliable antenna in the front fender, wiggle the base to see if the noise changes. If it does, its ground might not be making good contact with the sheet metal. If yo have a power antenna, those have a ground strap attached to the base. Be sure that isn't corroded off.
If the noise definitely just started after the brake service, the suspension was allowed to extend fully when the front end was raised off the ground. Often there will be ground straps attached to the struts' upper mounting plates, or between the lower spring plate and the lower control arm. Check if one of those was just about corroded through, and extending the suspension was the final blow that broke it.
Monday, December 5th, 2016 AT 3:36 PM