Is it a hum, a growl, a rattle, etc.?

If it's a metallic rattle, the fact that you can hear it more easily in
lower gears suggests to me that the catalytic converter heat shield is loose
or rusted.  This happened to my '93 Accord earlier this year.  If you don't
drive or park on grass, just remove the shield and leave it off since the
part costs a few hundred bucks to replace.