I've made the experience that the volume of a N900's FM transmission seems to be lower if it is transmitted on a frequency that has a lot of white noise. This usually happens close to the frequencies of regular radio stations, whether they are actually broadcasting a program or not.
My guess is, that the receiver surpresses white noise and a portion of the N900's signal with it.

So my suggestion is to chose a frequency that is at least 3 or 4 MHz away from any regular radio frequency. In some regions with a dense radio network this might be hard or impossible to reach.