Originally Posted by PiccoloNamek
The microphone is an ear-worn mic that is pretty close to his mouth. We're recording in a large sanctuary in a church (My job is to record and edit sermons for broadcast on FM radio). It is extremely quiet in there as long as nobody in the audience coughs or something. I record at 44.1/16. The average level of the sound is around -4 to -6dBFS, and there are no peaks over -1dBFS.
My first course of action would be the position of the mic capsule. Moving the capsule an eighth of an inch can make a big difference with those mics. Try to get it as close to the mouth as you can without touching and without being directly in the air flow. The second point is that churches are always difficult to mic properly, the usually large volume of air and highly reflective sufaces do cause quite a high noise floor, even if it appears silent. Brownian motion can have an influence in churches. Although, a good lavalliere should combat this reasonably well.
Is the receiver unit reaonably close to (say within 15ft) and in clear line of sight of the belt pack? Do you change the batteries in the belt pack after every service? Is there a high level of RF interference in the area or others using similar RF frequency devices? UHF are almost always better than VHF models. Diversity systems can provide a great improvement if the subject is moving around but if they are stationary and the receiver is well placed there shouldn't be much difference.
If after playing around with the mic position and eliminating the above you still can't get a much better SNR the only real alternative may be a tighter pattern or better quality mic and possibly a higher quality transmitter and/or receiver. A cheaper alternative might be to use noise reduction, although good noise reduction can be more expensive than a brand new high quality radio mic system. If you have to use noise reduction it's best to do two or three passes at light settings rather than a single pass with stronger settings.
This is probably the most advice I can give without actually being there to see and hear the problems for myself.