Had a similar sound coming from my SE Hybrid no nav package, although with the radio off. Personally would have described the noise as light scratching or feedback static, but if it had been much louder the sparkler sound could have applied.
Not very loud and went away (or to quiet to be decernable) when radio turned on. Never isolated where exactly the noise was coming from and seems intermittent at best, since I've been unable to make it happen on demand. My concern was it might indicate a short somewhere in the speaker wiring, but have not had much luck nailing down if it's coming from one/all speakers, initially thought it could have been some kind of vibration / road noise as it seemed to coincide with ICE on and a certain RPM range when puttering along on the last leg of my return home, from what little troubleshooting I've been able to do the 2 or 3 times I've thought I heard it.
Kinda wrote it off as a mysterious Gremlin I'd only have to chase down if it became a regular occurance, much more noticeable than the vague scratching noise I 'thought' I heard a few times, or fell victim to 12V battery issues.
My thinking is:
1. Possible EMF interference from a specific RPM / split mode combination, either from some kind of ignition timing sweet spot or power levels on MG1 or MG2 running close to idle easing back into the neighborhood. Although I would expect this to be more of an issue with AM reception, not creating some kind of static coming from speakers with radio off.
This thought is due to issues with my 94 Ranger and AM radio. Engine on vs off would seem to weaken the strength of the incoming station, although climate control fan speed seemed to cause the most interference when traveling and starting to lose the station. Hi speed fan would crackle much more than the low speed setting. Engine off would eliminate all interference and give the strongest/clearest signal depending on station. (Lights on/off seemed to change things as well, but think that had more to do with AM stations reaching farther at night, cant recall if there was something in the radio that changed to allow you to pull in those far flung stations at night with the lights on.)
2. Some kind of harmonic vibration from that particular low RPM / low load state.
(Seemed to be coming from close to the windshield on one/both? sides and to vary with gas pedal input or disappear with ICE off)
3. Speakers bleeding some kind of odd capacitance that had accumulated in the circuit or left over in a capacitor somewhere after turning radio off.
4. Biggest fear - Some kind or transient short in the speaker wiring.
Since it was present with radio off, primary concern was that if there was a short, 12V battery drain while parked would be an issue. Mainly just from paranoia over the other battery issues from MFT not properly shutting down, etc.
Haven't really thought about it in a few weeks, like you seemed like a bizarre freak occurance. So will try to remember to keep an ear out for it. Only seemed to occur for me at night with radio off and ICE on for me, but may have still been there when I turned radio on as well to check. Was barely audible to the point I wasnt sure I wasnt just hearing things or possibly hearing some faint 'new' engine noise that the hybrid makes that would eventually be an audible que, like the manual shift point hums of the old Ranger.
Edited by SilverSurfer, 09 November 2014 - 04:32 PM.