The pressure released after a bullet leaves the muzzle will be lower with a longer barrel generating less noise. When unsuppressed the 18" barrel should be noticeably louder than the 22". With a suppressor mounted the longer barrel should technically still be quieter but the difference between the two lengths would be much less pronounced.

Michael, this is relative and will depend on the actual ammunition powder burn rate. Powder tends to be burned more completely in a longer barrel which would naturally lessen the muzzle report under both suppressed and un-suppressed barrels making the longer barrel even quieter in a 22 inch barrel compared to an 18 inch barrel. Not all subsonic ammo is powdered that same so test for which is quietest in your 22 inch barrel. Cheers and stay safe!

Found a good thread on reddit that goes into details,
reddit.com/r/longrange/comments/5fw43z/loudness_and_barrel_length/?utm_source=BD&utm_medium=Search&utm_name=Bing&utm_content=PSR1
Doesn't seem like the decibel level difference would be much between 18" and 22"