[keithandpenne]

A couple of things come to mind, loose mounts or compressor working hard due to high load from dirty coils.

Is this something new or has it always been loud? Is it a metallic noise or just a vibration type sound? If metallic, it could be internal in the compressor, if a vibration, it may be a loose mount to the roof or duct.

I haven't been in ours so not sure what access is available to the coils but if they are dirty or restricted it will make the compressor head pressure go up. Also, running the fan on high speed in hot weather will reduce the load the compressor pressures.

Dust, pollen's and other airborne seeds can collect on the condenser and dust that passes your filters can clog the evaporator. Both can be cleaned with chemical cleaners but you would probably have to remove the unit from roof to do it.

Also, you can check your ducts with a mirror to make sure they have not been squeezed together by the ceiling insulation and causing airflow restrictions.