i have 225/40 R18 and was wondering if i could put a thicker side wall on them to reduce road noise a tad. Can i do that?

i mean taller btw

yes if you reduce the width a bit

use an online tyre size calculator to see what you can use

Try this http://www.rochfordtyres.co.uk/tyrecalc.asp

Aero, that's a maybe. You need to find another 18" tyre with the same overall diameter and a taller sidewall, such as a 205/50R18, if such a thing exists.

Unfortunately, even then, less tyre roar isn't guaranteed, since tread design has more influence on this than sidewall height does. Try and see if the last Autocar or Auto Express tyre tests are on their websites, if you want to do this, but note that you might find that you have to compromise wet or dry grip and/or braking distances.

What you will achieve is a softer ride, at the expense of more undamped suspension movement, and less overall dry grip.