I actually don't add salt to my food and watch the guidlines on products I buy, so I think I can rule out salt....unless that is what is in me ear!
The possible causes mentioned were:
1) Loud environment as DCStoxx suggested but I haven't been around anything loud for a long time.
2) Recent illness (Cold, Flu) - I suffered the flu in November last year so it's a bit of a delayed reaction if that is the cause
3) Ear infection from swimming - I got a very bad ear infection from the local pool but this was about 1 year ago at least.
4) Flying - Highly possible because I took a long haul flight early this year and a short haul flight about 1 month ago. But flying has never caused a problem before.