Radio Noise in 2004 GP

OK folks, I need a little help. My daughter's 2004 GT, 3.8L, with stock AM/FM/CD Monsoon radio system seems to have an issue. She gets intermittent noise/humm/static that is audible in the two drivers door and center dash speakers. Noise is of varying degrees of intensity, can be heard in the mentioned speakers with the radio on or off, and usually the noise varies with the engine RPM (whine intensifies as RPM increases, decreases when RPM decreases). Sometimes the noise starts with no provocation, sometimes it starts when you shut the drivers door, sometimes the noise starts when you hit a sharp bump in the road. The noise usually, not always, stops when you shut the car off and re-start it. I've tried to open and shut doors, lock and unlock doors, activate electric windows, cruise control, traction control, turn signals, etc. to see if they affect the radio noise. They don't. Recently though it seems that the noise will stop immediately when you blow the horn. Problem is intermittent enough that I have only tested the horn twice but I'm 2 for 2. I haven't yet taken the radio out or taken the door panel off but think that's within my abilities. Looking for any advice anyone may have with this. I might mention that this problem seemed to start about the same time she started getting traction control/steering assist warnings that were recently solved by the dealer replacing a wheel speed sensor. Probably not connected but seemed coincidental. Dealer speculated that the amp might be going out but that really doesn't seem to make sense to me do to the intermittent and non repeatable nature of the issue.

One thing to check is the radio antenna module......have seen that cause problems.......usually an intermittant problem caused by a bad ground....the antenna ground may not be tight(simple bolt that attaches the module to the roof).........I believe this module is located at the rear edge of the headliner, in the center, maybe 2-3" in.....there may be 2 plastic retainers you have to remove to pull the headliner down in the rear, just enough, to get a wrench in there to tighten the bolt....

If it's not there, it might be behind the driver's side rear panel, that goes from the top of the seat to the headliner........

Also check the connections to it......

Thanks for the suggestions. I guess I know what I'll be doing this weekend! I'll report back what I find.