If it is the same symptom you describe, I have a similar issue. Only happens around 2,000RPM ish and only under load. In other words, sitting on the driveway and revving the engine to 2,000RPM does not cause the 'rattle'. I thought it was the heatshield but it seems fine. The car did have problem with some gravel being stuck between the shield and the pipe which caused vibration but they reckon they fixed that. However, I spoke with a Honda dealership and it seems there is also another heatshield 'over to the left'. We concluded it could be some sort of harmonic resonance. Not sure where this other heatshield is exactly but I will have a look next time it is up on a ramp.