You know anything about ignition noise?

So I bought one of these little Bluetooth/FM transmitter dongles a few months ago.  It works fine in the suburban, fine in the Passat, fine in the turbo Miata.  But in the bone stock 2002 NB I get significant buzz/whistle/noise through the speakers.  It seems related to RPM but not directly and not exactly evenly (at certain RPMs it fades instead of getting constantly higher/louder).

Given that we're about to head out for an epic roadtrip, I'd like to be able to use it without the noise.   Any suggestions?

What's kind of weird is that we had a different brand of those FM transmitters kicking around here a while back.  Worked fine in the GM-based motorhome, was fine in the Ford truck, but in the NA Miata and the Mazda 5 it did exactly what you're describing.  The 5 was worse than the Miata, but I have to wonder if Mazda does something weird or different.

Two sources of noise. One will be in the electricity generated by the alternator and hence the feed to the bluetooth device to power it. The other would be in the FM band transmitted to the car radio. I am going to guess the first is the issue.

You could try powering the bluetooth transmitter directly with a 12 volt source  ( independent of the car's electrical (and see if it goes away.

If it does not clear up running on a battery then try moving the transmitter around to different places and see if it changes anything.

You could also try adding an additional (separate) ground from the accessory port.  RFI is often attenuated when given two different-length ground sources.

My guess is that since all vehicles have differently engineered ignition systems, the one in the Miata is just causing RF interference.  Nearly always, the culprit is ignition.  I remember back in the day if you switched to solid-core, non-resistance spark plug wires, you were just better off removing the radio to save weight.  You were never going to hear a radio broadcast as long as those wires were in there.

Hmm.  As we cross the country I've noticed some things.

First, the car has some pretty nasty belt squeal when it first starts up.   I'm thinking the alternator is due for replacement. Because:

The whine seems related to both electrical load and engine RPM.    Particularly the blower fan and the headlights.

Odd thing:   Using the Pandora app, the whine is just barely noticeable.   But using the Libby audiobook app it's really nasty, almost painful.

No help from me.  On my 1998 4Runner I get ignition noise when using FM radio.  Bluetooth is fine.   I've replaced plugs and wires, alternator, antenna and stereo head unit.  I also added a power filter, a filter from the radio antenna, grounds to the antenna and stereo body.  Nothing has made any difference.

In general us and german cars seem the least prone to this and asian ones are worse. The miata grounding schemes are, from an electrical fundamentals point of view, poor design. I redid some things on a mild miata build to follow the "bosch" style grounding for the ecu and gained 2whp. 
 

that all being said, find another nb at a car lot to try it and if it still happens its design endemic. If not likely a poor ground or ignition component, alternator, etc.