Humming Sound between 40-45 and 50-55mph

I have a 2004 Saab 9-3 Linear with 68K.

I am hearing a humming/rumbling sound between 40-45 and 50-55mph.  If
I hit the brakes to slow down it goes away and if I accelerate out of
it, it goes away.  It sounds and feels like I am in too high of a gear
for the speed that I am going (if it was a manual).  All I have to do
is accelerate past 45 and 55 and it goes away.

What do you think?