If it's a stock Ion there is no On/off ASA. If it has an aftermarket ASA there may or may not be an oring which can leak depending on the brand. With the stock ASA there isn't alot of complexity to it which can leak. It's more likely the leak is in the macro line or the tank oring. You will want to make sure all the bolt and regulator orings are good also. There is also several hoses and banjos where you can be leaking from aswell as the solenoid. I would suggest you break it all down and replace all orings first as that's the most likely problem and the easiest fix. If it requires banjos, hoses or solenoid repair you can find most items at the following:
A Freak kit is your basic Freak Barrel kit containing boring inserts, barrel back and front. That would have nothing to do with your leak issue. Freak kits are available at that link aswell.
If the marker is functioning and the leak sound goes away I can't imagine it's a major issue and you can find all the information for any maintenance you require online and on this site.