It's flex sections failing as said above.

I was getting the same vibration / noise on my OE exhaust at 1000 RPM exactly since purchasing my car (got it pre-owned). Gutted the cats (retaining otherwise OE exhaust) down the road but noise / vibrations remained. In about 6m thereafter changed the flex sections (there was an audible leak from them) by cutting the OE ones out and welding the new ones in their place (same size as OE ones) and the noise / vibrations are both gone. It's been over a year since and everything is fine by date.

Make sure to use "interlock" type flex sections (which is braided mesh on outside and harded "corrugated" section on the inside) as they are stronger / hold longer than cheaper all-mesh flex sections which are also available.