I think its the propeller condition lever that alter the pitch of the propeller which makes it sound like the RPM is changing.Its very noticable on air craft like the Dash8 and ATR,especially the older models,I think on newer models[Q400 and ATR-500 its computer controlled and the change in pitch sound is less prominent.]
I think its hard for the pilot to synchonise the RPM of each engine on the trislander so they are in harmony with each other cause when I have travelled in a trislander the engines seem to be all over the place.
maybe someone can give a more technical explanation
Regards.