Atlasman,
I noticed the same thing on my MQ1. Indoors, ninja quiet, but outdoors, a slight twang. I' m figuring that it has something to do with air pressure or humidity, or something like that. I' ve also noticed that outdoors, some days it seems to be quieter than other days. I' m not going to blow out a brain cell trying to figure it out, though. I use catwhiskers and I have one leech on the cable. The cable leech does make a difference. I' d had people stand 15 yards away, to the side of course, and they say that they can' t really hear the bow go off. It works for me.
Good luck,
Charlie