Assuming modern jet aircraft with APUs running, it will probably be in the 90-100 dB(A) region. It's difficult to be specific as there are a lot of variables.
At the top end of that scale, daily exposures would probably be limited to 5-10 minutes for H&S reasons without hearing protection.
It will likely be dominated by higher frequency noise which ear defenders and ear-plugs are most effective in attenuating.
My preference would be for foam ear plugs. Cheap and very effective.