Heat shields?
Improperly secured exhaust system?
:insert random component name here:

All I can do given the description, is guess. Best bet would be to put her back together, hold at the desired RPM, and probe about the engine compartment with a stethoscope. Be sure to check from underneath as well as above!
After you isolate the area from which the noise is likely coming from,
then start taking it apart.
