Noise from exhaust

[chewy]

I've had a new rear exhaust fitted recently. It's started to make a noise like a rattling heatshield with a high pitched squeak. I've taken it back to garage which cured it for a couple of days. They said the noise was coming from the springs by the cat that holds the bolts.

Anyone have any ideas what can be done to stop the annoying sound? It only happens when idling. The sound stops if i put my foot against exhaust, or if i feather the throttle slightly.

TIA