B Pillar/Rear Quarter Glass Rattle

[Blizzak]

Hello guys, been dealing with this weird metallic-ish rattle coming from the seat belt receiver/rear quarter glass area for a LONG time now. Searched to and fro on this forum and from my research over the past months nobody quite has this exact problem.

I attached a video with this sound... this has been happening with my STOCK exhaust and my now aftermarket one. This happens only at speed(60+ mph), there is no sound at city speeds(10-45 mph). Also this same sound is happening on the passenger side in the same area but that one doesn't rattle all the time.. and when it does it's very faint.

https://www.youtube.com/watch?v=RqMpzMCMezA

Today I removed the rear plastics and drove with them all off... noise was still there. Pinpointed it where my suspicion was... b-pillar/rear quarter area. Also previously took the exterior b-pillar cover off and the noise was still there. Running out of ideas... I'm thinking I may have to buy a whole new seat belt receiver. Or could this be a heat shield under the car?

Any help would be appreciated, getting desperate