What "certain gear combinations" do you mean? I am assuming that the chain is rubbing on the side
closest to the frame (???). That is the commonest one. What bike do you ride? That can also make
a difference.
On most bikes you should not use the gear combos that place the chain on an extreme angle. So
small front / small back is a bad combo. You should change the front to the middle and lift up a
couple of cogs at the back to get the chain back in line, with roughly the same gearing.
In fact my bike (Giant Yukon) recommends that I don't use the smallest _two_ cogs on the rear with
the front in small, and the largest _two_ cogs on the rear in the largest front cog. So with the front in
1, I should not be in 7th or 8th on the back. I should go to 2 in front, and maybe 5th or 6th in the
back.
If however, the chain is rubbing in any rear gear with the front in the _middle_ ring, then it needs
adjusting for sure. Middle gear front should allow the full range of rear gears.
Whatever, you may need to adjust the front derailer. It ... _should_ .... not rub in any gear combo.
But if you cannot adjust it to stop rubbing at extreme chain angles, without causing gear change
problems, then use the front gears more, and make sure that changing gears is smooth.
Gear change problems will be most obvious when trying to move to a larger cog on the front, which is
when the derailer has to push the chain up to the cog. If the chain _is_ rubbing on the side closer to
the bike frame, then you need to adjust it so that it _just_ misses when you are in smallest front /
smallest rear. But as I say, if that causes gear change problems when going up in size, stop
crossing the chain rather than adjusting the setup.