Can you drive to warm it up (so it makes the noise) then stop & make the noise by bouncing the car? If so, then do that while looking down there to pinpoint the location.
Or since it's under warranty, you should be able to get the dealer to do just that. If not, maybe call Honda USA & ask why the dealer won't check it out...
Do you have paperwork from the dealer saying you brought it in for that particular problem? If so, then I think you should be good for warranty because you brought it to their attention before the warranty expired.
'05 Saab 9²x aero (RIP)
2 '07 Civics
'09 Saab 9³ss
'10 Saab 9³sc