Speaker ZZZTTTT every 3 seconds

[Teepee]

Hi Gang,
97 Ford explorer XLT. Stock Stereo.
in the last couple of days my rear right hand speaker has started to make a ZZZZZZTTTTTT sound every 3 seconds.
I thought maybe a speaker going bad so today I took the door panel off both rear doors and switched speakers but the ZZZZZZTTTTT sound continues to come from the right rear position.. So I assume the speaker is not the culprit.
I would love some ideas on where to look next..
Does the fact that it is every 3 seconds give any hint at all???
thanks
Tony

 

[jakemccarthy]

It sounds like some sort of interference. do you have a cb radio, bluetooth phone or headset, or scanner?

I have a bluetooth enabled head unit and every few minutes there is an awful buzzing sound coming from the subwoofers. Bad enough that I shut the bluetooth off.

 

[Teepee]

Hi and thanks for looking.
I don't have bluetooth but I dohave a cell phone. Just in case, I switched the cell phone off today and still zzzztttttt every 3 seconds.
It still is the rear right speaker.
I'm just trying to figure out what sends a signal or interference every 3 seconds. That may help work out what is causing the sound.
Thanks again for looking
Tony