Would certainly recommend reporting this to your maintenance provider so they can investigate. Any drop in torque and unusual vibrations could indicate deterioration in the engines and if that is the case its only going to get worse.
I am not familiar with this aircraft exactly, could I ask if there are any keep out zones for the engines, this could also be a possibility if there is. The FCOM should clarify that though.
If your interested check with maintenance the engines Total Time, Time Since Last Shop Visit and Time Since Last Hot Section inspection. If the issue is internal of the engine they may be approaching a defined service limit that will require a shop visit or perhaps there is some premature deterioration if you operate in a particularly harsh environment. Also check that the compressor turbine blades and power turbine blades are being replaced, on the PT6 the life limits for them are contained within a Pratt Service Bulletin rather than the engine life limits where the disk lives are.