Odd resonating sound - backbox?

Hi chaps.

Bit of a weird one which has been around for a few months now. At exactly 2200rpm I get a droning sound coming from the engine area. Now this goes away above or below the rev range. Had it on a ramp today to take a look and mate thinks its the exhaust backbox. Has anyone experienced this before? He knows more than me on this, but have not seen this before. Exhaust and heatshields look to be secure and in decent condition. When the engine is held at the revs above and you hold the tailpipe you can feel this vibration.

Any thoughts?

Ta

Edited 28 March, 201214 yr by Liverpool-Lad