"scratchy" radio transmission

I am looking for ideas here; ATC says my radio transmissions sound
'scratchy' or 'staticy' as little as 15 miles out on callup. I have
been going nuts trying to find the cause. The aircraft has 2
independant radios, 2 antenna systems (one on top of the fuselage, one
underneath). One radio replaced as an update recently. Both radios have
been checked out by the factories recently and pronounced fine (one
Narco Mk12D and one Icom). Been through the wiring and grounds several
times and found nothing. Tried different headsets (note: the voice
sideband through the headset sounds perfect). ATC said no difference
with alternator shut off. Reception is always perfect. Complaints seem
to pick up more in the cold weather. Don't know if its important but
maybe once or twice each summer I get alot of static in the headset
that seems to disapate in a minute or two, but never in the winter.
If I were generating a large static charge in the dry winter air
wouldn't it effect my reception? Some days I get complaints even when
taxiing for takeoff. Any ideas what to try next without going on an
expensive fishing expedition?

Jim