Hi Sam,
Sounds like Tinnitus to me. Quite a common thing that most people will experience. More info on it can be found here:
Tinnitus - Wikipedia, the free encyclopedia
My advice would be to turn that blooming iPod down!!
If you suffer from allergies or have a buildup in ear wax then try to remedy them. A nasal spary is bliss!!! (not in the ear!!) I suffer quite badly from allergic rhinitis and a quick burst of one of these is awesome! You might have an ear infection which will require some anti biotics. An ear infection isn't nice at all. That sounds like you have a cockroach crawling through your ear canal. It can be painful too so if you start experiencing pain and / or have the sensation of fluid in the ear a bit like after swimming then see a Doc ASAP.
If you're struggling to hear out of one ear then I would wait until it's cleared. You will need to do a hearing test so you don't want to be half deaf when you do it. I believe the CASA Class 1 is very similar to our CAA/JAA one.
You shouldn't fail your class 1 unless you hearing is permently damaged. Listening to that iPod at 90db all day long will do you no good at all.
If it is just tempory however then you should be fine as soon as it clears up.
My best advice; check with a doc. That's why they're there!
Have a Bonza day mate.
SHG.