Rear speaker buzz every 3 secs

[Teepee]

Hi Gang,
97 Ford explorer XLT.
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 pretty well exactly every 3 seconds give any hint at all???
thanks
Tony

 

[Teepee]

Just a quick add on to the above post.
This noise only happens when the stereo is powered on.
I have the stock stereo system in.
Please , any body.. I am having to sing to myself.. Not Pretty
Tony