@Joselito,
What is the length for your 1:16 liner? If it is longer than the 1:18 liner, then a difference in velocity and noise at the same settings (regulator pressure and hammer spring preload) is to be expected.
In theory, the only difference between liners is the rifling twist rate. The 1/16 spins the projectile faster that the 1:18 at the same muzzle velocity.
Individual barrels may have different friction, so the velocity could vary due to friction differences.
In theory, a faster twist rate may slow the projectile down in muzzle velocity, because there is more energy in the projectile spin. For slow rifling such as 1:16 there is very little energy in the spin, so I would go more with friction differences; especially at the muzzle choke.
This assumes that all the sealing O-rings in the barrel system are in the correct place, and undamaged. Else, simply changing the liner may have caused an air leak, which would reduce performance. It may also release more noise from the breech of the airgun.