Rub: The noise is normal for an external reactor. It is annoying, but it is not a sign of a problem. Check out this thread: https://www.aquaticplantcentral.com/f...=reactor+noise
Also, to dispel a couple myths in this thread: there's nothing inherently bad about putting a reactor on the input side. Debris is a problem in theory, but seems to be much less of a problem in practice.
One problem with putting the reactor on the output side is that it can reduce flow. Ghazanfar Ghori and I have both experienced this using an Eheim 2213. The location of your reactor should be decided based on your particular setup, but either side can work just fine.
Putting the reactor on the output side will not get rid of the noise.
The noise you hear is not CO2 building up in your filter. Also, CO2 bubbles are probably not going to hurt your Eheim. Eheims are designed to prevent airlocking, and many hobbyists inject the CO2 directly into their Eheims, using the filter as a makeshift reactor, without any trouble.
In short, don't worry. If you can't stand the noise, you might consider an internal reactor. As you'll see from the linked thread, I have tried to find a solution to the trickle noise on the external reactor and never figured anything out. Wish I could be of more help.