MALCOLMX-8

I have been experiencing a low frequency rumble type vibration when moderately accelerating at 3500-4500 rpms in 3rd gear and upwards.Seems to have started after a small rear end accident(some #!*%! jerk ran in to my one month old car) wich damaged the muffler and middle pipe .These of course have been repaired but I am now getting this intermitent vibration.It never appears in first or second gear,has nothing to do with engine temperature and seems to disappear at higher rpms. The dealer technician confirms that the vibration does not seem normal but doesn't know what's causing it.The exhaust system seems to be installed OK according to them. Anybody else with a similar problem or any suggestions?