I've got a similar noise at the moment but it is caused by bumps / poor road surface not engine revs. Is it still likely to be exhaust hangers or heat shield?
Just looked under the car, one of the bolts for the lower heatshield has degraded/snapped off, and it is definately the culprit, my advice, when rattling occurs under the car, especially when cold, check the heatshield. I know my solution... de-cat!lol
Pretty east fix tbh! Just get some of that metal wire I posted a pic of on page 1 of this thread and wrap it round the exhaust till its tight then crimp it.